Setting up the chain tensioner is easy – just shorten the chain so that, when the bike is unfolded, the two tensioner wheels are roughly in line with the tensioner arm screw.
Belt tensioner setup is not much more complicated. The kit will come with the correct length belt for the frame and hub combination you have – the dropout on the right side can be pulled gently apart far enough to squeeze the belt through, then fit the sliding dropouts.
On the left side, the same bolts hold on both the sliding dropout and the disc brake mount. On the right side, there are two M6 bolts with locknuts on the inside, and a M5 nut. With most combinations, the correct position will be as far back as possible in the slots – if you’re not sure, leave the bolts slightly loose and do a test fit.
The tensioner then goes on (see the Alfine or Rohloff fitting instructions for details). The tensioner has a snubber roller that should be adjusted to press the belt onto the rear cog, it can slide in its slot if you loosen the bolt, so after fitting the tensioner, loosen this bolt half a turn and push it as far down (assuming the bike is upside down) as possible.
Then fit the front belt cog, it normally fits B-side out (the cog isn’t symmetrical). With the Rohloff hub, you normally have to add 5mm of spacers under the right bottom bracket cup to move everything out a bit, for perfect belt alignment. The Alfine setup needs no spacers.
The toothed roller should run smoothly in the groove on the belt. If needed, it can be pushed slightly left or right to get it perfect.