Basics, basics, basics.......get them right.
The amount of candidates I saw last marking season that didn't check equipment either before KO or when substitutes entered play was almost into double figures, which is poor. Likewise, managing substitutions......don't just stand in the middle of the pitch and wave them on.....make some effort to control the process.
Sock tape.........no excuse for not getting this right.
Undershirts/shorts......on a Sunday morning grassroots level, you can show some pragmatism about it......but probably best if you mention it before the observer does.
Talk to both teams before the KO, explain you are being observed and that because of that, everything will have to be pretty much letter of the law......even though you should be doing this at every game, it may help build a little rapport (sympathy) and give you some slack from the normal grief.
The amount of times I hear players moaning 'you're booking me cos he's stood there'.......after they've hoofed the ball away 40 yds on a FK.
Don't get too hung up on learning the Assessors handbook and trying to make sure that you are ticking those boxes as you referee the game......you will spend more time worrying about whether you've covered a competency and probably miss something that will cost you. Just referee your normal game....it's blatantly obvious when a referee is changing their game just because I am stood there, and it rarely does them any favours because they are not used to it.
Remember, the observer isn't there to catch you out....for most situations they are simply looking to understand your thought process for making a particular decision, rather than just hammering you for not doing what they would have done.
Get the mandatory cautions/dismissals right, get the process for cautions/dismissals right.
I am more interested in hearing why a referee did something (or didn't do something) than I am in whether it was the right decision. If it is a major error, then I can give suitable words of advice that should prevent them from making the same error. If it is something that is poor practice then I will try to give the referee another way of looking at the situation that might lead them to another solution.
Above all, it's about developing referees, and you cannot do that if you only ever criticise without offering an alternative. Likewise, the candidates need to be prepared to hear some things that they might not like about their game, and be prepared to make changes. If they are not, then it is likely their promotion quest is doomed to failure as well as wasting everyones time.