Once you've made your box you might find it to be a little tight initially until the material relaxes into its new box form.
For boxes with a double crease fold on a long edge:
Particular care needs to be taken with the double crease, it is essential that the crease closest to the interior of the box (shown in green) is folded first. In the pictures, our operator works their way along the double crease using the edge of their hand to make sure the innermost crease is folded.
Once the tabs are tucked in and we bring the box together the second crease will naturally fold over, this should give the double folded edge a nice square profile.