Here's a tip for some easy paint masks.
Draw your design onto a piece of paper or find a design from on the net and print it onto a piece of paper at the size you want.
Next get some "Contact" Self adhesive Vinyl which is available from most department stores or stationers, (I use the black one)and stick the design onto the back of the Contact Vinyl Sheet with some adhesive.
When its dry cut out the design of your paint mask and there you have it, a ready to use vinyl paint mask.
Make sure you press down firmly around the edges and paint immediately after as it can lift a little if left too long.