Snape wears robes that have special sleeves that flare out a lot at the arms and are split up to the shoulder.
Make the sleeves extra wide, then split each sleeve up the side after sewing your robe together.
Potion Master Jacket
The black jacket that Snape wears under his robes has a high collar and reaches down to just past his knees. The large black buttons start at the neck and end at the bottom of his chest. There are also buttons from the elbows to the wrists on the sleeves.
You can simulate the potion master's jacket by turning up the collar on a black robe and adding buttons down the front. Use safety pins to gather the robe at your back so that it has a tighter fit. For the sleeves, add buttons from elbow to wrist, making sure that they gather the fabric of the robe when fastened.
Underneath the potion master jacket, wear a white collared shirt. This should show a little underneath the unbuttoned high collar of the jacket.
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.
Snape wears black square-toe boots that are usually covered by his long black pants. This is good news for you because you won't have to worry about what the tops of the boots look like. Find any pait of square-toe boots at the thrift store. Apply black shoe polish if they aren't the right color.
Snape uses a polished black wand. It has a few inches of carvings on the helm, but the rest is smooth to the tip.
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.
Wear your hair in a shaggy black cut that is parted down the middle. I suggest smoothing a touch of mousse over the top of the hair to slick it down and add a slightly greasy look.
Dying your hair black is easy because all hair types will take black color well. Henna hair color is one suggestion, because it is gentle on your hair and typically lasts only 4-6 weeks.
This tutorial was taken with permission from about.com's DIY Fashion section.