Online solving 4 tactics puzzles is a good warm-up.
Over the board the good warm-up is to think about the first opening moves, as if you do not know, just to get into a state of concentration.
Do You Warmup to Play Chess?

I'm still figuring out what might work for me. Coffee, some puzzles, and some meditating seems like the minimum for me.
How do you like to warmup?
and
How do you know when your are ready?