Sewing up to finish collar

the directions start by saying "Sew 1 of collar to right side of front neck opening (it will reach approx 2/3 of the way across) .Sew other end to left side, lapping over right side. Slip-stitch edge of collar to neck of sweater, being careful not to pull to tight.
I have done the sewing part but am confused on the slip-stich the edge.

The first directions are for the ends of the collar which overlap at the front. Once those are sewn, the remainder of the collar is attached to the neck opening by sewing it with the slip stitch (this is the sewing version of a slip stitch not the knitted slip stitch). I think a whip stitch which is close to the same thing is better or just overcast around on the wrong side to attach the collar to neck opening.