There is a sticky that Maggie3fan wrote. It is called how to measure a tortoise, or something like that, at the top of these threads. Read it. It has pictures and detailed instructions. That is the way I do it.
I do it this way: Place them face first into a wall or similar flat level object. Make sure their shell is touching the wall or whatever it is you are using. Take a stick, rock, or pencil and make a mark in the dirt where the back-most tip of the tortoise is, and use a tape measure to measure the distance from the wall/object to the mark you made.
Put the tape measure up to the wall, on the floor. Place the tortoise up against the wall with his head retracted. Read the tape measure at the back of the tortoise's shell. This is called SCL - either straight center line or straight carapace length.