Size the sheet. Cut off the sheet's top hem - the area that would be near your face if you made your bed with this sheet. Fold the sheet in half, and cut an opening for the head at the center of the fold. Assuming this is for one-time use, that's all you need to do for the opening. But if re-using or concerned about raveling edges, use a fusible web tape from fabric stores - following its directions, or stitch around the edges.
Make it the right length. With the head through the hole, mark and cut the bottom edges of the sheet so it ends just below knee length. If you're very tall or want the Indian costume to be longer than knee-length, you may not need to cut it off at all.
Turn your sheet into "deerskin." Unless you already have a very dark brown sheet, this process will make white, off-white and tan appear aged and more like leather. Fill a large kettle with enough water to submerge your sheet entirely. Bring to boil, remove from heat, and drop in the tea bags. Allow them to steep until the water is quite dark. Remove bags. Completely wet your sheet and stir it into the brown solution. When you get the color you desire (realizing it will be lighter when dry), remove and rinse it with clear water until no brown comes out. Dry by itself in the drier.
If you plan to wear a tee shirt under the Indian costume, stain it at the same time.
Make fringe. Around the entire bottom edges, cut ½ inch (or so) wide slits, about 3 inches long.
Make the headdress. Measure and cut enough raffia to tie around the head and stay in place once knotted, letting some surplus dangle after the knot. Though once on the head, feathers can just be tucked in place, they stay better if their ends are actually glued to the raffia first and allowed to dry, either upright or downwards, back and sides if you wish. So glue on and dry the feathers now if you choose.
An option is to thread on, glue on or sew on a few beads as well.
Add berry stain art. Along the bottom edges, right above your slit fringes, draw on a zig-zag pattern in red with the red fabric marker. Make each zig and zag about 3 inches.
Make the belt. Cut enough raffia or cord to tie loosely around the waste.
Wear it. Put on the tee stained tee shirt if using it. Gals may instead want to wear a tan or brown leotard. Boys -- an already dark colored plain tee shirt. Slip the deerskin over the head and tie closed by wrapping the raffia or cord belt loosely around the waist and tying with a loose square knot that's easy to undo. Put on the headdress.
Gals may want to wear tan or brown tights or leggings under the garment. And, extra feathers can be pierced and wired to regular loop earrings. Guys might want to wear very plain khaki shorts or pants.
Brown, black or leather flip-flops or sandals go well with this Indian costume.