Draw the shape of your skateboard deck on paper to make a template. Tracing is probably the best way to do it but make sure you bend the paper where the board curves to get the right shape.
Sketch out your design on the template. Draw anything you want but make it simple enough that you will be able to recreate the design with paint. If you aren't a very good artist, you can trace images you like with tracing paper. Just make sure you aren't stealing someone else's skateboard graphic.
Make stencils for the various elements by cutting the shapes out of cardboard, tag board or other thick paper that will hold its shape.
Cover the board with a base layer of spray paint. This will be the background color.
Wait for the first layer of paint to dry. You must let each layer of paint dry completely before adding the next layer.
Place your first stencil onto the deck and spray paint the cutout area. It helps if you tape the stencil down so it can't move while you are painting. You should also make each stencil from a large piece of material and cover the entire rest of the deck so you don't get the wrong color paint on the exposed areas.
Continue adding layers of paint, letting each one dry until you have achieved your desired design.
Finish your design with a layer of varnish to protect the paint, but be aware that the varnish can make the deck a bit more slippery.