I've just bought 3 small gelli plates in different shapes - round, triangle and square. They come in a pack and looked like fun! In the first photo you can see where I've pressed the round gelli onto a perspex stamp base. You can buy these on line, and they're useful as they have grid lines for registering and wavy edges to make them easier to hold.
Rollering some paint onto the plate. You just squeeze a bit onto the plate and use a brayer to roller until you have the texture you want.
Place a stencil over a shape and stamp with the gelli plate using a different colour. It's easy to line up as you will be able to see the printed circle through the gelli plate.
Just continue to stamp and overprint with stencils until you have something that pleases you. If like me, you are doing this in a sketchbook, it's a great time to experiment with colour and texture. Just go for it!