To install a mortice lock into a wooden door, and to get the right heights the door must be already fitted to the frame. This makes aligning the lock and striker plate easier, you can do this using a tape measure and transferring the dimensions across, but that can lead to errors. 

To hold the door in the right place use a wedge or door-stop to prevent it from swinging on the hinges as you are working on it.

For the best results the mortice lock should be fitted in the middle of the cross rail, this is where the door is strongest. and will have the most material to work with and secure the lock in place. 

This will also put the door handles at a height that works for most people.