Approximate Method
First find an approximate value for the focal length.
This is done by projecting the image of a distant object, e.g. a cloud, onto a screen, e.g. a sheet of white paper. The distance between the screen and the back of the mirror is approximately equal to the focal length.
Precise Method
Apparatus
Raybox with cross threads, screen, concave mirror, mirror holder, metre stick.
Method
Set up apparatus as in
diagram.
Move the mirror until there is a sharp image of the cross threads on the screen.
Record u,
the distance from the cross threads to the back of the mirror, and v, the distance from the cross threads
to the back of the mirror.
Repeat for different values of u and v.
Graph 1/u against 1/v.
Draw the best straight line.
From this line choose a single value for 1/u and for 1/v.
Calculate a value for f using the formula 
Sources of Error
- The image of the cross-threads may not be sharp enough.
- The error or parallax when reading the metre stick.
- Measurements may have been taken from the front rather than the back of the mirror.
Solutions
- Perform the experiment in a dark room.
- Ensure that your line of sight is at right angles to the metre stick.
- Take measurements from the back of the mirror.