The procedure for Adjusting belt tension is not particularly hard, but does require a specific sequence of steps. The only tools required are a 15mm box/open ended wrench (or adjustable wrench), a 6mm allen wrench, and 3 or 4mm allen wrench. The early versions of the bicycle used 3mm and the recent version uses a 4mm.
There are lots of tools available that are meant to help set the tension on the Gates Belt. Aside from the factory-level tool, we can't recommend any of them. We've found that we get better results just setting it up like a chain. In general, looser is better, so long as the belt isn't skipping. We like to see 1/2" of deflection in both directions at the middle of the belt when pressed lightly. (1" total deflection)
First loosen the axle nuts and verify that the wheel axle is all the way in the top of the drop out slot. This is done by resting the bicycle with wheels on the ground (perpendicular to the ground as if you were riding) and pushing down on the rear rack before tightening the axle nut to secure the wheel. If the wheel is not butted up against the top of the drop out slot, the belt tension will change every time the wheel is removed for service.
If the axle slid upwards during the above procedure, then you may actually be done at this point, assuming that the belt tension reads within the proper specs.
If the tension is not right after seating the axle properly the procedure is as follows (refer to picture at the bottom):
1. Loosen sliding drop out bolt A and B on both sides of bicycle
2. Starting with drive side/right side (side belt is on) turn the tension bolt until belt has achieved proper tension. Use your preferred method to check tension-app/krikit/deflection amount.
3. Tighten the non drive side/left side tension bolt until wheel is centered in frame-double check tension to verify it is still correct
4. Tighten down sliding drop out bolt B on both sides of the bike
5. Loosen the tension bolt slightly on both sides of bike-2-3 turns
6. Tighten down the sliding drop out bolt A on both sides of bike
7. Tighten down the tension bolt on both sides