Choose where you want the bolts. I usually have one on each side positioned so the window will open up to 4 inches. Check the fitting instructions to get the drill size right (it's usually a 10mm drill). Drill a hole just deep enough for the bolt housing. With large screwdriver screw the bolt housing into the frame until it is flush with the frame.
Tip to get the depth right when drilling wrap a bit of sticky tape on the drill at the depth you want.
Tip You should find when screwing the bolt housing into the window frame it's a tight fit. If you find it very difficult use a little dab of washing up liquid to make it easier.