Step 1: What's Needed
1. A chair, of course.
2. A seat belt WITH ENDS ATTACHED.
3. Something to attach the seat belt to the bottom of the chair.
4. A brain.
1. A chair with threaded holes, hopefully with metal threads.
2. Two screws that fit the threaded holes.
3. Two nuts that fit the screws.
Step 2: How to Build It.
2. Get your seat belt (complete)
2.a. Check that the seat belt may come across your entire lap without strain. Adjust if needed.
3. Think up a way to attach the belt ends to the chair bottom.
3.a. Tape, tie wraps, nails, whatever works. Fuse it onto the chair, if you want!
4. Realize your lifelong dream of racing office chairs down hills.
4.a. Take out some medical insurance before you follow your unsafe dream.
4.b. Get spares of whatever you used to make the Seat Belt Chair.
4.c. Get some body padding.
4.d. NOW you may follow your dream.
4.e. Remember that starting is the easy part, stopping not so much.
5. You should end up with something like what I have below.