We drew the pattern for one leg by following the outline of existing PJ trousers. I increased the length a bit. It's important to stretch the elastic band at this stage or there won't be enough room for the waistband to go past a rotund bottom.
I placed the pattern on the folded fabric by the crease and cut, adding seam allowance. Old bedsheets - good re-use! I sewed the tummy-to-bum seam first (French seam), then each leg (French seam again), then each hem, then the channel for the elastic band.