These instructions are to supplement the Stained Glass Bubble Mitten pattern, although the method can be used in any case where you are finishing a doubled cuff (whether for a mitten, sock, sleeve, etc.).
Begin with cuff worked to twice final length, with provisional CO sts at the bottom (in these photos, the waste yarn is dark brown and the provisional CO sts are light brown).
Fold cuff CO edge up into inside of the cuff and align the CO sts with those on the working ndl such that the ribbing is straight, not skewed. (Because you are looking at both ends of one ribbing, the CO edge as well as the sts currently on the working ndl, as well as both sides of one ribbing because it is folded in half, remember that a knit st at one end will have a corresponding purl st at the other end.)
Place a small number (10-25%) of CO sts (light brown) onto a provisional ndl and hold the provisional ndl behind and parallel to the working ndl, with both ndl tips pointing in the same direction.
You are now going to do what is akin to a three-needle bind-off, without binding off any sts: Working together one st from the front working ndl with its corresponding CO st on the back provisional ndl, *k2tog, k2tog, p2tog, p2tog* to the end of the round.
I prefer to leave the waste yarn in until I have finished joining the cuff edges. Removing it after the process is complete allows for the provisional CO sts to remain hooked to something; should something go awry in joining the two edges, the join can be easily undone with the provisional CO sts remaining safely on the waste yarn.
Here is a photo of the back side of the join, with joined sts on the left and sts to be worked on the right: