Round 1: Work one SC (or HDC) into each stitch around the edge of the blanket, working 3 stitches into each corner space. Join with a slip stitch at the end of the round.
Round 2: Continuing on in the same direction. CH 8 (or any number if you’d like your border wider or more narrow.) Starting in the second chain from the hook, work SC in each of the 7 chains back toward the edge of the blanket. Slip stitch into the next space, and in the next space. Turn and work in the back loops of each of the 7 SC you just made. CH 1 and turn. Work back down the SC in the back loops only. (Be careful not to miss the 7th, it sometimes pulls tight and is hard to see.)
In the first of the three stitches that make the corner, only slip stitch once, before turning and working the SC back up to the edge. Return and slip stitch into the corner space. Turn and work the SC back up to the edge. When you return to the corner space, Slip Stitch again into the same corner space, then work the SC up to the edge. Upon returning, Slip Stitch one more time into the same corner space for a total of 3 times, before moving on and only slip stitching one time into the next space.
TIPS: You may adjust the number of slip stitches in between rows if you are stretching or the blanket edge is rippling. You may prefer just doing 1 slip stitch instead of 2.
Here’s a video tutorial of the blanket this border is on. (Note, the first round is Herringbone Half Double Crochet instead of SC or HDC, which is fine to use if you’d like.) Skip to about the 12 minute mark for the border.