Maroon and Gold are the colors of Gryffindor house, and you'll need to show them off on Harry's tie, scarf and robe.
Start out with a gold tie. Use red fabric paint to add the lines on the tie. If you're using paint and a brush, employ a ruler to help you make straight lines. A fabric pen works much easier in this case and most fabric stores have them in many colors. Make two thick lines, then skip an inch and make a very thin line. Skip another inch and repeat this pattern.
Find a pair of glasses at your local thrift store, or make your own with the Print a Pair of Harry Potter Glasses tutorial available in about.com's DIY Fashion section.
Harry's dark brown mop top is easy for most kids to pull off. If you have lighter hair, there are lots of semi-permanent hair colors out there to try if you don't mind committing to brown hair for a few months.
Any thin stick can be made into a wand. Peel off any bark and use a piece of sandpaper to smooth down the sides of the stick. For a pointed end, use a small knife to whittle the end of the stick. Shaving the end of the stick should be done with adult supervision. Apply a dark or light wood varnish to your smoothed wand.
You could also use about.com DIY Fashion's Easy Pencil Wand for kids, or one of the many wand tutorials available on Leaky Crafts in the Woodwork and Other Crafts sections, or spring for a toy Harry Potter wand.
Look in your local thrift stores for a white button-down shirt and a black or gray vest to wear over it.
Wear a pair of black slacks, or make your own simple black drawstring pants, using the tutorial available on about.com in their DIY Fashion section.
Black dress shoes will work for your feet.
This tutorial was taken with permission from about.com's DIY Fashion section.