Sock patterns and mitten patterns both call for you to pick up stitches.
Top-down socks have a heel flap which usually has a slipped-stitch edge that you need to pick up into, whereas mitten thumbs have a raw edge.
Here’s how to handle both situations.
By the way, patterns call this technique “pick up and knit” and they abbreviate it “pu and k” or “p/u and k.”
If you liked this tutorial on how to pick up and knit stitches, post in the comments!