Something I see a lot at fields are people attaching MOLLE pouches like the post image. Running the straps just through the top and bottom loop of the vest/chest rig/LBE, and snapping them closed.
This creates a lot of potential for pounces to sag and shift around while you're moving. Pull mags out can be harder as the pouch sometimes wants to come up with it. Generally just not ideal. You shouldn't be able to fit your hand between the pouch and the vest.
The better way is to weave the vertical straps back and forth between the MOLLE ladder segments on the pouch and vest, getting as many points of contact as possible. This can be a little difficult sometimes with especially stiff straps, but a simple use of needle nose pliers can make even that a breeze.
Properly run MOLLE will look like:
No big gaps. The pouch and vest will act like a single item rather than a loose, shifting or hanging pouch pulling off of the vest.
