I've done this different ways. Paint the seat one solid color and then use a square sponge dipped in the second color and go across and down the same width of your sponge. You can use masking tape to hep make squares or after you've painted one color go back and use a ruler or something the size you want and make your squares to paint.

Hope this helps and you will continue to come here and post with all of us. There is such nice people here and everyone is always willing to help others. Please come back and show us a picture of your finished stool. Punk