Finishing off - At the Cuff
How to finish off the cuff edge when you have made your mittens top down.
Continue nalbinding until the cuff part is of suitable length, the way you prefer it.
The cuff part can be straight (about equally wide all through), a bit widening (mitten is wedge-shaped or A-shaped), or bell-shaped (widening more strongly). In the bell-shaped mittens, you can first decrease a bit below the thumb, and then start increasing. The decreasing below the thumb help the mitten to stay better on the hand, ie not fall off so easily, and also highlight the bell shape of the cuff. An outline-pattern may help when nalbinding a bell-shaped cuff.
Last row - how to finish off the last stitches
When you are finishing off the edge, you can first start by pulling the stitches smaller, and at the same time gradually pick up less loops behind the thumb, and the ones you have around the thumb (if you have more than one).
If you have tensioned the new stitches around your thumb, you can pull them smaller by tensioning them around the thumb nail/tip. When the stitches become so small the thumb loop does not fit around your thumb anymore, you can insert the needle through the stitches without thumb loop(s) (flat). Do a couple of those smaller stitches.
If your stitch has been e.g. Finnish Stitch 1+3 (Brodén Stitch), you can gradually pick up less loops behind the thumb. First, go with FS 1+2 (Mammen Stitch), do a couple of them. Then do FS 1+1 (Oslo Stitch), and do of couple of them, too. Then do FS 1+0 (ie 1 thumb loop, but no loops onto needle behind thumb), do a couple of them, and finally do a couple of stitches, like you were simply sewing by hand around the edge of the fabric.
In order to get a 'gently sloping' edge, do the finishing off little by little, during several stitches, and often it is good to make two or three stitches before you change the stitch (ie pick up less loops behind). At the end, you can pull the last few stitches (FS 1+0) smaller by pulling the yarn by hand (see the video), and finally 'sew' over the edge of the cuff a couple of times. Hide the yarn on the reverse side by weaving it into the loops there.