Usually you remove the spark plug and install a piston stop. This stops the crank turning and allows yo uto unscrew the clutch. BEWARE, usually left hand thread.
If you don't have access to a piston stop, put the piston to Bottom dead Centre, fill the cylinder with oil and replace plug. This will lock the engine up.
when clutch job is finished, remove plug, empty out oil and flush clean with fuel.
Allow to dry out for a while before re starting. May blow smoke for a few seconds after starting, then clear up.
