The snaps should really be in the center of your belt. If you have to put them off center then you have put the male snaps on your ab plate in the wrong position.
If you want to get it right first time then put the female snaps in your belt first, making sure they are in the middle of each end ammo block just as Chriss has above. Then you can offer your belt up to your ab plate and see where to put the male snaps. I put mine at 20mm above the split line where the ab meets the cod, and find that usually gets the belt in the right position.
Don't try and get your belt face too tight across your ab plate as it will make snapping it on hard. Plus, the belt does not go straight across your waist. It tilts upwards slightly so leaving it loose will make it look and feel right