With the Glock I have with me tonight, my procedure for that would be:
1: Use left index finger to eject depleted magazine;
2: Insert butt forward into the holster on my right hip;
3: Withdraw fresh magazine and insert about halfway into pistol butt;
4: Place left thumb over pistol's backstrap and use palm of left hand to crush magazine in until it locks (just slapping it in can pivot the gun out of the holster when gun is in there backwards, and leave it on the ground behind you);
5: Draw pistol with conventional cross-draw;
6: Use left index finger to depress slide stop and chamber a round, or run rear sight or edge of ejection port area of slide against belt to cycle slide if it has closed on an empty chamber;
7: Get back to shooting.
If using shoulder rig or otherwise without a hip holster, insert into left side trouser pocket instead of holster.
NOTE: YOU WANT HUNDREDS OF PRACTICE RUNS WITH DUMMY AMMO BEFORE PRACTICING THIS LIVE, AND I STRONGLY RECOMMEND THAT THE TECHNIQUE BE LEARNED UNDER COMPETENT SUPERVISION!