This was me a few months ago, until I managed to transform Sunderland into a top half side.
I played around with different formations until I found one that was good for me, but I did have a recommended formation from the assissman as a secondary formation. Using a 4-1-3-2 Narrow formation served me well for two seasons but eventually, the AI will figure your formation out.
I don't change formations when playing different games, but as a member above said, I often change instructions by looking at scouting reports for the opposition team. It can say where they concede the most goals / assists from. If it's the left flank, I'll put my LB into an attacking mentality and keep my RB back at all times, that way there is almost always 3 defenders back at any 1 time.
I've found after many seasons that too many instructions are detrimental. I only have about 7 instructions at the minute.
When it comes to in game instructions and advice from assistant managers, I take everything with a pitch of salt. You may get 2 half chances from crosses, which might prompt an AssMan to suggest hitting early crosses, pumping the ball into the box and using whipped crosses. If you apply this and tell your team to do it constantly (when previously they might have just been doing it because it was best to in that situation) you may well get another message 30 minutes later saying that your crossing just isn't working today. Through trial and error, you can sometimes see which in-match instruction work and do not work.
But as another member suggested, there are some great YouTube tutorials out there.
Hope this helped