Before you start, make sure the window is fully closed. Take your ruler and measure between the edge of the jamb and the window winder. Then mark the centre with your pencil. First, position a spacer over the mark and place the lock on top. The spacer helps create enough space so that the lock doesn't catch on the windowsill.
To mark out where the screws need to go, use a nail because a pencil won't fit. Push it through each of the lock's screw holes and make some small dents in the sash. Do the same with the spacer. Put the lock and spacer aside and mark those dents with your pencil so that you can see them better.
Wind the window open. Choose a drill bit that matches the size of your screws and drill pilot holes into the marks. Close the window again and place the lock over the holes. Screw the lock onto the sash with your screwdriver, but you can also use a battery drill.
Make sure the window is tightly closed. Insert the bolt into its hole and push it down to make a mark on the windowsill. Pull the bolt out and wind the window open. Now drill a hole into that mark but use a drill bit that's slightly larger. Close the window and check that the bolt fits into the hole.