Here is a tutorial on how to crochet around the edges of a lopi sweater that has been steeked (cut) – after having first made 2 seams to secure the stitches – to make a cardigan.
With the crochet hook, from right side, crochet an edging of US single crochet (English double crochet) along the cut edges of the steek: the crocheted stitch goes around the edge and hides the fringe (cut edges) of the steek, like a blanket stitch would do. We usually crochet in about 2 stitches out of 3 for a lopi sweater since the row gauge and stitch gauge are different. If you go in every single stitch then the edge will bulge.
Secure at the corner with a single crochet US (sc). **Then *insert hook in the middle of the next stitch along the edge, go through the fabric, yarn over, draw a loop, yarn over, draw through the 2 loops*. Repeat between *-* in the next stitch. Then don´t crochet in the next stitch but in the one after that. Repeat from ** to the end of the edge then secure with a single crochet US in the corner.
Here is a video: