Step 1: Materials
1- One bicycle (better with coaster brake).
2- Cranks (better short cranks for a child's bicycle).
3- Bottom bracket cartridge bearing, the sealed type, not the old system.
5- Two bicycle chains.
6- Some plastic tube to guide the chain.
7- Metal tube, about 25 mm inner diameter.
You may take items 2-4 from an old bicycle. I bought a long stem because the one in the bicycle was a bit too short for me. I also bought special tube for chains because it is very cheap, but you may use any plastic tube of suitable diameter.
Step 2: Bearing
Fix the cartridge bearing to the stem clamp. If the housing of your bottom bracket bearing is not strong enough it may be dangerous, the bearings are under the threads and the clamp of the stem is holding the housing at the middle. Do it on your own risk.
My housing is made of a very hard resin, I think it is OK. I cut the frame of the bicycle to take it out because I don't have the special tool needed to dismount it.
Step 3: Cranks
Fix the cranks to the bracket. To dismount the cranks from a bicycle you need a crank extractor and a crank key. It's very easy, explained here:
Step 4: Handles
Cut the plastic of pedals and polish to give a cylindrical shape.
Cut two pieces of metal tube and pressfit the pedals inside.
I made a hole in the tube and inseted a small nail for more safety.
Step 5: Front Wheel
Mount a rear wheel in the front fork. You have to expand the front fork, for instance making some leverage with any kind of tool if your frame is steel. An aluminium fork may be expanded using a dismounted hub as in the photo.
I mounted a coaster wheel in order to have a front brake.
Step 6: Chain
Join two chains and guide them from the crank to the wheel using the tubes. The path of the chain in tension should be straight, the returning path doesn't.
You need a chain tool for this work. The use of this tool is explained here:
Depending of the rotation direction you want, you may cross the chains (like me) or don't cross them. Probably the most efficient way is not to cross, like in hand bikes.
If you do not like the tubes, you may use an old dérailleur to guide the chain.
Step 7: Finshed
That's all, the work is finished.