THe crankshaft and camshafts must move in specific time with each other for all of the other mechanical parts in the engine to operate in the right sequence to make the engine run. ALigning everything correctly and installing the timinig belt is known as "timing" the engine.
This can be a little complicated to do, and is the hardest part of assemblking an engine. You may want to have your mechanic friend complete this part of the assembly. If you want to try it yourself, here is the procedure:
I hope this is helpful to you! If so, an accept would be most appreciated! thanks!