The chain guard comes with two brackets: (1) a seat stay bracket and (2) a down tube bracket. Each bracket has a nut and a screw. Note that the nut for the seat stay bracket is usually already pressed into the bracket, but it occasionally comes loose in shipping and needs to be placed back in the seat stay bracket.
Wrap the down tube bracket around the down tube with the holes for the screw facing down and the nub that attaches to the chain guard facing the chain ring. Put the nut and screw in place, but keep the bracket loose so it can be adjusted later.
Wrap the seat stay bracket around the seat stay. Make sure the screw hole faces down and the side of the bracket with the nut is on the inside of the seat stay (towards the wheel).
Insert the nub on the down tube bracket into the slot in the front of the chain guard.
Position the hole at the read of the chain guard inside the seat stay bracket an insert the seat stay bracket screw.
Move the brackets and chain guard until the chain guard is properly positioned on the bike. tighten the bracket screws.
Note that it is not usual for the crank arm to rub against the chain guard. This is a result of the narrow Q-factor combined with the dual chain ring guards.