HI,
One method described is to lie on you tummy...then imagine that your spine is like a bow...and try to approximate your both ends of the spine ie;your head and coccyx..but dont let the movement occur..i mean the effort should be isometric...you can ask your friend to place his fingers over your low back paraspinal muscles.So that you will get feedback once the muscle contracts...
second method ...is an advanced one..where lower limb movement is used to recruit multifidus...
Lie on your tummy...place your legs slightly apart and rotated outward.now slighly bend your legs and press the posterior aspect of heels together...slighly lift the bend knees off the ground..say;10cms still maintaining the heels pressed together...hold for 8-10 seconds..then slowly bring down the legs...release the pressure between the heels..once the leg touches the ground..
hope it is helpful
Cheers
Linbin