Make a Gryffindor scarf by crocheting, knitting or felting. Note that the knit scarf pattern has thick blocks of color instead of the thin lines of the Gryffindor scarf that the older students wear.
Start out with a gold tie (compare prices). 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.
The Gryffindor Robes
Make your own Gryffindor Robe in any size. I takes just minutes with this easy tutorial:
Finding a pair of round glasses at a thrift store is pretty easy. You can also substitute the these print-out glasses for the real thing.
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 (compare prices) 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.
I suggest looking in a few local thrift shops for these simple elements. Find 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. Black dress shoes will work for your feet.