With you in the bathroom, shut the door and hold the bolt against it in desired position.
Hold the part (name?) which the bolt slides into, on the adjacent door frame.
If your lucky the door and frame will be level with each other, ie. neither will stand proud of the other, and the bolt will cross the gap.
In some cases you may need to use a chisel to remove a bit of the frame so as the bolt will slide across.
When they are level, holding them in position - bolt first, mark through the screw holes with a pencil.
You may want to make a small hole in each 'mark' to guide the screw, but as they are small you don't need a drill.
If everything was aligned properly, you should again be able to hold the parts in place and simply screw in one screw at a time until the two pieces are in place and the bolt slides easily...