Wherever you see a 28mm diameter hole and approx 8 mm deep, there goes a 6001Z ball bearing (28mm OD, 12mm ID, 8mm wide), Theres a total of 10 required. 

2 on top of every crank.

2 on the side of every pedal.

2 on the back lever.

every pair of ball bearings is held in place with a M12 threaded bar.

You probably think that 10 ball bearing is a lot, but placing them on opposite sides of every pivoting point ensures that theres not latteral play on any of the pieces.

Dont hesitate to ask any questions you might have or, even better, suggest changes that would make this any better.