Measure and mark the desired side height. I chose 60 cm.
Take out your angle grinder (or a saw) and cut the folded edges on either side, so the countertop will fold easily. Allow for the metal on both sides of the cut to overlap (see third picture).
Fold both ends of the countertop upwards (with the countertop lying upside down). It's easier than it looks. I did it by hand and I'm no Wonderwoman. I just stood on it, pressing a piece of wood down with my feet, and pulled the end upwards with both hands. Easy-peasy.