I've seen too many stripped Mk.16/17 selector plates on VFC SCAR's, and even experienced techs have trouble with the selector mechanism. So here's a pic of how to get it right the first time.
The lines are there for a reason, so line them up. That will set everything in semi auto. Drop it in the gearbox as is, you don't even need to put the right side selector gear in yet, you can slide it in after you've confirmed selector alignment.
Hope this helps.