Horizontal with the opening facing the buckle. I only saw 1 other guy carry this way in my many years. It's what I have done since the academy and the only way I am used to. Tried the vertical but doesn't feel good and many of the "cool" dudes here also carry horizontal but face the opening of the pouch outward. That is actually almost as common as carrying it vertically.
I use my left thumb to hook the flap and undo the snap button and grab the downward facing magazine out in 1 sweeping motion from out to in and come up with the left index finger on the front of the magazine butt plate which makes it intuitive to insert the mag into the magwell. Very fast for me but it's all muscle memory.
Cool story, bro... when do you get to the part where you shut up and walk away from me?