Find the right car seat for you. You should be able to pick up a car seat at your local junk yard. The seat will cost between $15 and $100. You may choose a seat that retains its power adjustments. (This will require more work when mounting the chair.) Or you may choose a seat solely for its comfort. Choose a seat you can imagine sitting comfortably in for hours at a time.
Bolt the chair to the plywood. Measure the base of the chair. Using a table saw, cut the plywood to the same size. (You can also get the plywood cut to the correct size at your local hardware store, usually for free.) Cover the plywood in upholstery fabric to match the chair. Finally, bolt the chair to the plywood piece, making it as secure as possible.
Mount the chair to the base. Take a base from an old office chair, and carefully place the car seat on it. It may take some adjustments to find the correct balance. Once the chair is placed properly, screw the chair in place. Now your office chair is ready to roll.