Lots of good information here throw the kit instruction in the trash. When you get to a problem you don't understand ask here. People on THIS forum like to help a new guy.
I have a boy go fast kit and I know the instructions are worse than useless. they can be dangerous. Read the threads here then ask if you dont understand dont guess.
Amen to that!!!
Much of the install is based on "gut feeling". Start with the splayed out parts inside a large but shallow tub or box or whatever, so nothing gets lost but everything is visible.
I'd say mount the sprocket first to dip your toes in the pool. Then the throttle and clutch onto the handlebar only. Then place the engine within the frame to see what angle of mounting "feels right" allowing access to spark plug and air filter. If the engine mount doesn't lie flat against frame, do enough filing so that it does. (And did you remove all the studs and apply loctite? You should.. you need to, you'll regret not doing it). Don't tighten the engine in the frame, just snug enough to adjust position without it falling off, so that it can be aligned best with rear sprocket and throttle and cables etc, by tilting engine as needed.
THEN, refer to posts regarding proper throttle and clip assembly and washer position, Norman's carb disassembly thread, clutch adjust thread, READ READ READ!!! And remember that performance will be less than stellar until about 200 miles break in is finished, so you can only tinker so much to make it better until then (I think BikeguyJoe's quote...If it ain't broke and you keep messing with it, it WILL be broke! = paraphrased)