Materials:

Two tires (don't go stealing these off your parents car) — you can use old tires or new ones that you get from an auto body shop
Old seat belts or a strong woven material of your choice, like canvas or rope, preferably of two different colors
Wood
Measuring tape
Screws
Drill
Staple gun
Bolts

Step by Step

Step 1
Clean the tires
Hose down the two tires with soap and water. Use a scrub brush to get the dirt off, and if there are any remaining marks, use a surface cleaner. Make sure these dry completely.

Step 2
Build the frame
Next we need to build a wooden frame that is a little wider than the opening of the tire (you won't see the wood when you place it in the rubber of the tire). You will need an adult's help for this. You'll need four short pieces of lumber, plus some smaller triangles of wood to put over each corner of the frame and secure it into place. Your parent or guardian can help you measure the tire and wood, then use the drill and screws to secure it together.

Step 4
Measure the seatbelts
Now we're ready to figure out how many strips of seatbelt material you'll need. The strips should be about 6 inches longer than your frame.

Step 5
Attach the seatbelts and get ready to weave
First, lay out all the same colors of the strips side by side going one way over the frame, so there are 3 inches hanging off each end of the frame. Using a staple gun, attach the seatbelts to only one side of the frame. Then do the exact same thing on the side next to the one you just did (at a 90-degree angle).

Step 6
Weave
Now weave the seatbelts into a crisscross manner. You'll lay the seatbelts over and under each other like in the photographs. When you are finished weaving, ask your parent or guardian to help you attach the seatbelts to the other side. One person needs to pull the fabric tightly so that it is taut. The other person should use the staple gun to attach them to the frame. It should look tight and neat!

Step 7
Insert the seat into the tire
Lay one of the tires on the ground, and cut slits around the opening on one side. The slits should be on the side that you won't be sitting on, or where the two tires will touch when they are stacked up. Insert the seat into the opening through the slits.

Step 8
Bolt the seat into the tire
Ask your parent or guardian to bolt and screw the wood into the tire; the rubber is very thick and will hold a bolt without ripping. We recommend using a rounded stainless-steel bolt., as it won't cut you when you sit on the tire.

Step 9
Bolt the tires together
Screw the two tires together from underneath the bottom one.