How to avoid or fix those recording mistakes that make it to the mixing stage......
As a general rule, you'll always hear "get it right at the source" and "don't rely on fixing something in the mix" proclaimed by all and sundry working in the audio business. An they're absolutely correct.
HOWEVER, in the real world of low budget or DIY recordings we often don't have the time, equipment, resources or knowledge to get it right at the source. When it comes time to mix you tracks you realise there are some serious errors in the source material which greatly effect the sound. Things like:
Phase issues
Clicks / pops
Mistakes
Excessive leakage on microphones
Poor tonality or sound quality
There are a few things you can do during the sessions to try and cut this off at the pass to combat this. There are also a few things you can implement during mix-down to salvage as much as possible.
The best case scenario, which is the first thing a Facebook group troll would shout at you is "re-record the track". This is wonderful, however not usually at all helpful as you can't always get the artist back, they might not be able to re-record for a number of reasons or the time scale involved would throw everything into disarray.
To avoid or mitigate these issues, try the following:
During the Session (Pre-Emptive Arse Covering)
Give yourself the best monitoring options you can. Monitor from outside the recording room where possible and check the recordings as much as time allows. Check mic positioning between takes to ensure nothing has moved and let the band know to watch for any mic movement, too.
Pay attention during playback. Get the musicians to understand their responsibility towards identifying performance issues. Get them to dial in and listen rather than sitting noodling on a guitar (I'm often amazed with how often musicians pay little attention to their or others' takes). If only one instrument is being recorded, listen to in solo for errors, or solo with a key instrument (Like bass guitar against kick drum) to ensure timing is tight removing distractions from other instruments can be helpful in identifying these sort of errors.
Don't rush setup. You're going to be against the clock but make sure you've left enough time to set up what you need to or plan to use what you have enough time to set up!
Don't fatigue the players. Keep the drummer fresh and the singer's voice intact by not overdoing rough run-throughs or repeating takes of sections they've already nailed.
Hide the Jack Daniels from the guitarist and also stay away from a social drink, toke or whatever yourself. Everyone is less likely to make mistakes and more likely to spot them (or be inclined to do anything about them) when you're all fresh. Granted, you're not a babysitter so there's limits to how far you can control the band but you have a responsibility to the client to be fresh and responsive yourself.
Record a couple more takes than needed. This gives you more options for comping the tracks together. Just be wary of tiring out drummers and singers.
Use room treatment. I don't mean necessarily hundreds of pounds of gear from GIK or B&Q (although better is obviously better, just not always required). If you're in a poor room, throwing up some packing blankets or duvets (or even winter coats!) can soak up some reflections which can improve things like drum overheads, hi-hats and vocals. Sound bounces off untreated walls which can cause phasing issues and general tonal imbalance.
Record drum hits of each kit element prior to recording a track. This is super useful for missed or inconsistent hits, which can then be replaced by the sample. Get the drummer to play each drum repeatedly and slowly with increasing volume so you have a full library of hits. You can also multi-sample later to make a sampled drum kit if you're that way inclined.
Take a dry sound of the guitar before it hits any pedals or amps. Connect the output of the guitar to a DI box r amp sim box and then take the DI box output to your interface. The "Thru" output of the DI can go into pedals / amp / amp sim for the guitarists usual tone. This uses an extra track but gives flexibility if, for example, a effect pedal is jarring (e.g. delay not a right tempo to track), there are pops / clicks from switching or even it just doesn't sound good. You can the use amp sim and effects on the DI track to duplicate the original intention. This gives the option to create a second guitar track, too by duplicating the DI track with different amp sim and rearranging the sections (Verse 2 is at Verse 1, Chorus 2 is at Chorus 1,etc). The variations in tone and playing will sound like a second guitarist. Bear in mind that the sections need to be repeatable to do this.
If a singer is struggling with hitting notes try having them remove one headphone ear so they can hear their own voice directly. Varying different headphone levels can make a difference. Also don't shy away from (if you have time) recording phrase by phrase. I've seen me record vocals with someone by playing the melody through the headphones on a keyboard and recording a phrase at a time whilst they had the melody there to aim for (you could also record the keyboard I guess). One other technique is to auto-tune the closest recorded vocal and have them sing along to that, however you need to have and be quick at using autotune to keep the momentum going.
Check tuning repeatedly - don't just assume the band will do it. Get the band to use the same tuner and if at all possible encourage them to maintain equipment prior to the session. Things like guitar intonation, drum tuning and damaged cables, etc can all become huge issues during recording and mix-down if they're not nipped in the bud quickly,
Fix it in the Mix (Remedial Arse Covering)
Phase issues can often be addressed by aligning the waveforms within the DAW manually. Zoom up close and make sure the overheads waveform is aligned, then match the other mics as near as possible to this alignment. You won't get it 100% perfect but it can make a huge difference to the sound.
If you find you could be needing a doubled track that wasn't recorded for vocals or guitar, adopt the suggestion above and copy and paste duplicated lines for other sections. Just duplicating the track won't work as our ears and brain subconsciously know it's the same thing copied. There are "Automatic Double track" plugins and chorus effects that can have a similar effect, too though.
For poor drum recordings don't overlook drum replacement. Logic pro X has the facility to change or augment drum sounds to MIDI notes. Sometimes it can be fiddly but it can reap rewards if done right. There are also plugins like Drumagog, Slate Drums, Mdrumreplacer or Superior Drummer that have replacement trigger software which automates the process to varying degrees.
Editing can solve problems with pops / clicks / performance fluffs. This can again be fiddly but practice makes it easier. These days you can easily copy sections, phrases and even single vowel sounds. Just be aware that it's easy to fall down a rabbit hole with this and butcher things beyond the level you need to.
Pitch Correction is very prevalent these days and a DAW like Logic pro actually comes with it included. This is obviously handy for re-pitching vocals, but unless you're wanting it to be very audible, the vocal needs to be as good as possible first. It's another technique that is very useful, but can do more harm than good in the hands of the unpracticed.
An "emergency break glass" scenario for poor general sounding tracks is to make it sound intentionally bad and a "Character" of the recording. There's a charm to old 4-track tape recordings that people like, for example. This will only work in some situations and for some styles but I massively endorse it from the standpoint of it being better to use what you have than nothing at all.
If you have money to spare, plugins like Izotope's RX suite can deal with a lot of artifacts caused by less than ideal recording environments.
Some musicians can be a bit precious about these techniques, especially if they're old school purist types. You need to strike the balance between placating their artistic sensibility and getting the best sound. Things like drum doubling, pitch correction or recording guitars with DI can be the devil's work to some musicians and it's fully understandable, however in modern recording or recording with budget, time or equipment limitations, you sometimes need to think outside the box or make an idealistic compromise to get the sound you need. The final word has to rest with the artist though. It's their baby, you're just getting it over the finish line.
As I also alluded to above, as long as it's understood and taken as a stylistic choice by all involved from the outset, don't be averse to just embracing the limitations and making the statement that the final product was the "thing that could be achieved with what we had". Everyone has to start somewhere and often some really edgy and interesting sounds can be achieved by just rolling with what you have rather than following the pack.
Also, unfortunately some players aren't that great. People struggle with timing, playing consistency, pitching or technique. They can also have poor gear and limited to no idea how to set it up. More often than not at the DIY or inexperienced musician has never heard their equipment in isolation and are still learning the ropes. They're also still learning their trade. This might not stop them expecting to come out of the session sounding like the Foo Fighters. This is where the biggest problems can lie because if they're remotely delusional about their status the sound deficiencies generally land on the head of the engineer. Throughout these sessions be encouraging and positive, straight talking and professional. If you can demonstrate as you go ways which you've improved the sound or coaxed an improved performance it will likely be remembered. BUT very importantly don't patronise them or try to lead them down a path they're not comfortable with, especially if it's a first session. Treat it as fun then set some objectives to improve on for the next session.
Finally, remember that a lot of this is down to planning, preparation, communication and observation. It's not necessarily something that requires extra equipment. Even if you do need to polish something, most of the tools are readily available in the average DAW. Aim to minimise the polishing required by taking your time during the recording process and not getting carried away. Identify issues quickly and act on them. Decide early if there's a level of "grunge" everyone is willing to accept due to circumstances. Determine what is realistically required to mitigate against any issues in future sessions. Primarily, do your best to make sure everyone enjoys the session. They could be return customers.
Happy polishing (or not polishing).
.Foll.