Buy a wood board of any size you want. Wood panels that you can buy at an art supply store usually come in maple, but you can experiment with different types of wood, if you want.
Cut the panel into the shape you want. Using a jigsaw, you can make your panel into a square, rectangle or an irregular shape. Make sure you take a pencil and draw your shape onto your wood panel before you begin cutting.
Sand your panel, even if you did not cut it. You need to make sure your panel's surface and edges are smooth before you continue.
Use artist's gesso to prime the painting surface of your panel. This will keep the water or oil in your paint from getting into the wood fibers. It will keep your panel from peeling or distorting over time.
Paint your wood panel using acrylic or oil paint. Of course you will want to wait until your gesso has dried before you start painting.