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
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
Step 4: Handles
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
I mounted a coaster wheel in order to have a front brake.
Step 6: Chain
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.