What to wear for Ron: This works best if you're a redhead; if not, you'll need a red wig. Wear black trousers and a cape over a white dress shirt, a grey vest, and a tie. Make a wand.
What to wear for Harry: All the above except the red hair. Pick up some round-framed glasses, and draw that infamous lightning-bolt scar on your forehead.
How to act: Practice some spells, but don't mention He Who Must Not Be Named.