Pick up 6 stitches from the right facing side of the cardigan that you cast off at the top of the front.
When you start to knit pick up one stitch from the back of the cardigan.
Knit this picked up stitch together with the first stitch.
Continue picking up a stitch and knitting it together with the first stitch. See how you have attached the collar to the back of the cardigan:)
When you reach the other side of the collar, cast off.