How to finish an open edge with the invisible fasten off
For amigurumi pieces that'll be sewn onto another piece but with the edge still visible, there’s a slightly smoother way to finish the piece. Like the regular fasten off, once you’ve finished your last stitch, cut the yarn to make a tail. Remember to leave a tail long enough so that you can use it to sew this open piece onto another piece.
How to do the invisible fasten off in crochet (step-by-step with photos)
For pieces that’ll be sewed onto another, or for a piece where the edge will be seen, you can use a technique called the “invisible fasten off.” This technique makes the edge look smooth.
 1. Finish the last stitch and cut the yarn so that the yarn tail is at least 6 inches longer than the circumference of the rim of the piece. |  2. Pull the hook away from the piece until there’s no loop left on the hook. Thread a needle with the yarn tail. |  3. Skip the next stitch. Insert the needle under the top loops of the stitch after that, from the outside to the inside of the piece. |
 4. Before pulling it tight, insert the needle under the back loop of the stitch the yarn tail came out from. |  5. Pull the yarn tail tight enough so that it’s the same size as the other stitches along the rim. |
How much yarn should you leave? It depends. If you’re planning to hide the yarn tail, which is called weaving in the yarn tail, leave enough so you can comfortably thread a needle and weave in the tail. If you’re going to attach this piece to another, leave enough yarn to do that and weave in the remaining tail. The way you weave in the yarn tail depends on if you're finishing a piece crocheted in rows, or finishing a piece crocheted in rounds.