SS1,
The way I did it was take a small phillips screwdriver (subsitute small punch, ballpoint pen, or other similar object as desired) & depress the detent in the bottom of the mag. While pressing it, slide the base in the direction of the rounded end of the base. Be careful, the mag is obviously spring loaded. I simply kept pressure on the plate inside (whatever its called) & slid the mag extension over it until I heard the reassuring click when the detent snapped back into place. Voila & repeat.