Take the free end of the timing belt and feed it around the stepper motor pulley

Continue along the underside of the Y-axis

Feed it around the Y-axis idler pulley...

...and continue back to the Y-axis carriage

Pull the belt and mark where it meets the Y-axis carriage. This can be with a marker, or simply by holding it.

Measure 16mm, or count 8 teeth on the belt, from the position marked previously. Double check your measurements and then cut the belt at this point

The belt can be cut with regular scissors.

It is better to cut the belt too long than too short! If you are unsure, cut it longer than expected, you can always remove more if needed