Hello ladies and gentlemen! The credit for this tip goes to the fabulous ladies here at the site. I wanted to convert a pattern for a regular pair of jammies to one seam. Deepika had already posted a tip about it previously but I just couldn't figure out how to add a taper to the legs. Georgene and Barbara Hatfield both gave great advice: the side SEAM becomes the new straight of grain. that is, overlap the seam allowances on the lateral side seams. Then overlap the hems to create the amount of taper that you desire.
Now, if your pattern is shaped in the hip area (as mine was) and you don't want to lose ease in this very important area (!!), use a pin to secure both pattern pieces at the widest point of the hip. The pin then acts as a pivot point when the hems are overlapped to create the taper. Clear as mud? When you are done, you'll have a crazy huge gap between the two pattern pieces. This is darted out when the pants are sewn up--just trace sewing lines along the edges of the pattern pieces. I trimmed the excess fabric after I sewed the dart. Next time I make jammies, I'll probably transfer the dart to the CF and CB seams.
I hope this was clear. I really enjoyed doing this pattern modification. And my jammies came out very nicely. (Picture is coming.) Woohoo!