Here's where I got the idea for the project, Piet Mondrain for Kids. They do a great job explaining how it works. I would recommend getting some of Piet's work that is similar to what you'll be creating to show your participants. Having a little bit of history on this gentleman is great too. You'll find info at his website here.
I first made a sample myself to show the groups. Then I premade all of them by just adding a couple of lines of tape on each page of card stock. Each one was a little different, I had them pick one that spoke to them. Then I would ask them where they would like to put the next line of tape. Once they felt there was enough lines, I gave them red, yellow, blue and black paint to use.
What I love about my groups is I try to give them a few rules to go by and they usually break all of them. I LOVE THIS. While I tried to have them stick to squares and rectangles there were many triangles and other wild shapes. They'd mix their paint to come up with other colors and THAT'S what I love about the creative process.
At the end of the class I like to ask each person to talk a little about their work. I try to see if they can come up with a title and any other thoughts or feelings they have about their work.