Place the frame in the position established when you measured.
Take a long screw and drill into one of the holes on the left hand side of the frame
Important - If you have brick or concrete walls you may need to first drill a hole in the wall and insert a suitable wall fixing. When done, you can then fix the frame screw into this wall fixing.
Repeat the process, inserting one screw, loosely on the right hand side of the frame.