Start by painting your pumpkin shape in your chosen colours. You may need more than one coat for a nice, even coverage.
On a piece of paper, print or draw the design you would like to apply to your pumpkin. It is very difficult to transfer a design onto the polystyrene, so make sure if is not overly complicated! When you are satisfied with a design that will work, draw it onto the pumpkin in pencil.
Pencil does not show up well if you are using metallic paint as a base, so you may need to paint a light outline onto the shape first before bulking up the lettering later.
Using a thin brush, paint your design using a contrasting colour. This may take a few coats, especially is you are painting a light colour onto a dark colour.