Fitting a barrel or sliding bolt is one of the easier DIY projects. Follow the instructions below.
Choose the height at which you want to place the bolt. There are no rules for this and now is the time to decide if you actually want two bolts, one at the top of the door and one at the bottom. Hold the bolt in the position you want it. Make sure the edge of the bolt is not over the edge of the door. Mark the position of the 4 or 6 screw holes with a pencil or bradawl.
Use a small drill bit to drill pilot holes for the screws and fix the bolt into position.
Place the bolt keep against the door frame at the same height as the bolt and mark round it. You may need to chisel out some of the architrave to get the keep back to the same level as the bolt.
Mark the screw hole positions, check the alignment of bolt to keep, drill pilot holes and screw up tight.
Another job well done!