I would recommend you read the instruction manual as you build.
I also recommend you follow along with the instruction manual, this way you can be sure no steps are left out.
Use the video as a reference only, use the instructions as the if it were The Holy Bible itself!
As for the head which is pre-assembled, you should take it apart, grease up the bearings, and apply thread lock to the screws which hold the blade-grips to the feathering-shaft. You should also add some oil to the feathering-shaft as you put it back in the head.
You'll have to disassemble all pre-assembled parts, and check them. The instructions state you should do so, and you should.
Good luck.