I use double sided sticky tape at the wrong side of the cardstock. It will keep the fabric in place when you work on the edge. Here I used a wide tape, but 2 strips of smaller tape works fine. Make sure there's about half an inch left between the tape and the edge of the ATC.
Mark the size of the ATC with a pencil on the backside of the embroidery. On this ATC I used a piece of batting. I attached it with the beads, after finishing the embroidery.
Attach front and back together with the help of the drawn lines and cut the extra fabric. Here I made my ATC sandwich and cut the bottom and the right side.
Then it's time to decide which knitting yarn to use. You need about 15 inch.
Work your edge counter-clockwise. Fold the knitting yarn in half and start with the loop 6 holes from the corner. Start with a knot in your sewing thread between the front and the back of your ATC and make 2 whipped stitches in every hole, starting in the 4th hole from the corner.
Make 3 stitches in the corners, 2 on one side and 1 on the next side. Hold the yarn in place with your thumb.
Continue until you've stitched the first 4 holes at the last side.
Cut one of the threads halfway between the remaining space . . .
. . . and put the other thread through the beginning loop from the back to the front.
Pull the thread through and bring it to the right. Hold it tight with your tumb. Make sure the thread you've cut halfway is inside the big loop you just made.
Make the remaining stitches and cut the last piece of knitting yarn.
To secure your thread nearly invisible, put your needle under a whipping stitch and wrap the thread around the needle a few times. Pull the thread through while you hold the wrapping in place with your thumb.
Put the needle next to the securing stitch between the back and the front of the ATC and come up about an inch from the edge.