I act as CAR for my local club most weeks. Find the ref pre-game and just say hi, I'll be running the line for XY United. They'll usually say come up with the captains before kick-off. That's it. No point mentioning you're qualified, ex-FIFA, or you invented the flag etc unless it comes up in conversation.
Listen to the briefing with the other lino - all standard stuff probably - whole of the ball out of play, which way the restart is, stay with second last defender, hold until they play the ball or are obviously blah blah... etc. Doubt any referee will want fouls from club linos, and I wouldn't be getting involved with foul throws either unless they implicitly ask me to.
Go, look pro, do what's asked of you, lots of eye contact and thumbs up, and enjoy a run around in the sun for an hour. Good for fitness and practising little things with very little pressure.
GO GET EM.
Oh, and don't wear ref kit.