You will need:
1 foam flip-flop sandal (I got a pair for a buck at Old Navy) be sure it's big enough for your knife.
a picture or some idea of what you want your knife to look like
a razor blade (optional)
a pair of scissors
duct tape or electrical tape
several feet of cord(or something else you want to make the grip out of)
Sorry about the lack of pictures. Once again, I didn't decide to make an instructable until the thing was already made. If I make another one, I'll put more pictures up.
UPDATE: Today (8-6-09) I went airsofting and wound up making 2 more of these for my friends. We wound up getting in knife fights all the time, and these work great! Also, I wrapped the new ones in electrical tape before cord wrapping the handles, and also put tape on the blade of mine. The tape covers the treads on the bottom, and makes iit slide in and out of the sheath more easily. I got lucky with the sheath, which I made out of the shoulder strap from an old messenger bag.
Step 1: Shaping the Knife
2. Use your marker to draw the outline of the knife you want on the foam. I recommend drawing it diagonally across the foam to get the most length out of it, and to avoid the holes from where the strap was attached. For my knife, I used a picture of a Ka Bar USMC combat knife, which is really freaking sweet.
3. Cut out your knife shape with scissors.
4. Wrap the knife shape with duct tape. I did not do this on mine because I don't have any duct tape. Once I do, I will perform this step.
Step 2: Making the Handguard and Pommel
2. For the hand guard, I took a scrap of foam about three inches long, and split it in half lengthwise. I then held a strip in either side at t he base of the "blade" and taped the ends together on either side.
Step 3: Cord-Wrapping the Handgrip
When you're done wrapping the handle, your done with your knife. Congratulations! Now go foam stab your friends, parents, pets, siblings, teachers, etc. (Note: Don't run around with this in public, you will probably get in trouble of some sort.)