Among the almost infinite variety of door locks out there, none combines openness with security quite like a swing guard lock. These are the locks that have a little metal arm hanging on the door frame, and it swings over a peg on the door face to lock. You have seen them in hotel rooms, and there's a reason they use them: They allow the person inside the room to open the door a few inches and peek out, just as with a door chain, except the swing guard is much stronger and can't be easily forced open. Remember, they only work on inward-swinging doors.
Stand inside the room with the door closed. Hold the two pieces of the lock against the door, interlocking, about a foot above the doorknob. The two pieces should be in the position they will be in when closed. The swinging arm should be on the door frame and close to the edge, so that the arm extends out over the doorway. The fixed peg should be on the door face, close enough to the edge that the swing arm can close entirely over it.
With your pencil, put marks on the door frame in the two screw holes on the base of the swing arm. Put marks on the door face in the four screw holes on the peg base.
Remove the pieces. With your drill and pilot bit, drill holes at each of your marks, about half an inch deep.
Put a screwdriver bit in your drill. Bring the lock pieces back up the door and screw them in place with your drill and screwdriver bit, putting the mounting screws through the screw holes on the base and into the pilot holes you drilled. Completely tighten the screws. Test the lock.
If the door doesn't close evenly and it's making it difficult to get the swing guard to lock properly, loosen the screws on whichever lock piece is inset too far, stick a wood shim under it, and tighten it back up.
Wear eye protection when using a power drill.