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.