How can I hold a plank for 1 min without trembling?

Practice!

Breathing and thinking about something else helps me.

Agreed with @YoBob practice!
Also you could try to listen to some music

It just takes practice to build the muscles you’re using. Make sure to squeeze all your muscles including your butt while you hold, and don’t forget to breathe.

“Ah Young Grasshopper…When You Can Take the Pebble From My Hand…You Will Have Six Pack Abs!”

Breath deep and silent. close your eyes to focus. Practice practice practice

The advice that I was given was to place a bolster between the knees. It works pretty well.

Start with 10 seconds, until you are stable. Then 20. Then 40. Then a minute. It is a process.

I make my trainer talk to me and tell me stories.