I looked on the web for instruction to make my own gift bags for Christmas. I found a site that had measurment and cutting instructions. I finally decided if I wanted the sizes I needed I would just get a box the right size and loosely wrap the end so the box would fall out and then I could fold it flat like a paper bag.
You do need heavy paper and make sure the pattern is upright. I made one with poor Santa standing on his head, which made good fuel for the woodstove.
They turned out fairly well considering.