First time painting on slate. What are the steps I need to know. Preparing the slate, type of paint, sealing?

Answer 1

I paint on slate all the time. No priming is needed. It's surface is porous, and accepts acrylic paint excellently. It's not recommended that you varnish the backside, due to it's porosity.

Answer 2

First why paint on slate? Then it is a matter of what will adhere, I doubt acrylic will without priming, which will cover the slate which I sense you want to expose, so I would say oil is the best route.