Casting on extra stitches

I’m making these Simple Mules for a friend and it was going along great untill I got to this part

The uppers (two the same) using 10 1/2 needles, cast on 19 stitches. Working the whole upper in moss stitch, increase 1 stitch at end of next row by knitting into front and back of last stitch, then increase 1 stitch at same edge on the next 5 rows for the toe shaping. (25 stitches on needle) Work 4 rows without shaping, finishing at straight back edge. Cast off 12 stitches in moss stitch at start of next row. Decrease 1 stitch at end of next row by knitting last 2 stitches together. Decrease 1 stitch at start of next row by knitting first 2 stitches together. [color=darkred]Increase 1 stitch at end of next row by casting on 1 stitch after last stitch. Work 1 row without shaping. Cast on 13 stitches at end of next row (25 stitches on needle).[/color]

and I don’t understand how to cast on stitches on a row you’ve already been working, just can’t figure it out.
Can anyone explain this for me, please?

When I have to cast on in the middle of a row, I use Backward Loop

I like to use knitting on when I have to cast on extra stitches. Video here: http://www.knittinghelp.com/knitting/videos//cast-on/knitting-on.mpg

Thank you very much!