The chain guard comes with two brackets: (1) a rear 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 rear 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 bracket around the chain stay. Make sure the screw hole faces up and the side of the bracket with the nut is on the inside of the chain stay (towards the wheel).
Wrap the 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 rear bracket an insert the rear bracket screw.
Move the brackets and chain guard until the chain guard is properly positioned on the bike. Tighten the bracket screws.