There's already feedback from much more experienced divers than me, but what the heck, here goes.
The DIR way is to use a simple loop crotch strap, dead center, just long enough to fit your waist strap though, without pulling it down at all. Say you're on the surface of really choppy water, you might want to inflate your wing fully to keep the water away from your mouth, which will make the rig a lot more buoyant than your body. The crotch strap will keep it from riding upward. Assuming you're not diving with a scooter, that vertical stability is the whole idea here, so just tight enough to dissuade your rig from riding upward.
The buckle goes on the right side, approximately 1-o-clock position. This means that if the buckle is accidentally loosened, the crotch strap will be a little bit in its way, and hopefully make it easier to put it all back together without digging around for the 3 loose ends. Conversely in the case of intentional removal, the buckle will still easily slip though the loop without fiddling.
Personally, I did not realize this when I set my rig up, and just assumed that the buckle should also go right in the middle, so that's what I did. It has not caused me any problems, and works just fine. I keep meaning to move the buckle a few inches to the right, but haven't bothered to yet. One of these days I'll give it a shot and see if it makes a difference.
By the way, there was another thread on this recently, discussing putting the buckle on the left side (whether attached to the right side and adjusted, or attached on the left side). This config variation seems to have many downsides and few upsides.