Pull the loose ends of the belt tight.

The teeth of the belt should catch on the bolts head holding the tension temporarily.

If you find it slipping way too much, tighten the M4 bolt a little more and pull the belt again, pulling the belt with the pliers will allow you to get a higher tension.

Use a pair of pliers to hold the Nyloc nut as shown in the second image and tighten down the bolt.