How do I cast on after I've knitted 24 rows?

Help! I am making a throw which starts out on the diagonal, increases in the middle and then decreases. The instructions read knit 24 rows stockinette then “cast on 12 stitches at the beginning of the next row, knit to end of row.” Cast on 12 (more) stitches, knit 12, purl 12, knit 12. I am totally lost. I have no idea how to cast on after the project has started. I know how to increase, but the instructions specifically say “cast on.” I’ve tried increasing, and tried to figure out how to cast on without starting with a slip knot, but it’s not working. I’ve been out of the loop for about 20 years and just started again.

I always use the backward loop method in these situations. You just get to the point where you are supposed to cast on, then cast on the number of stitches using this method. There is a video that shows how to do it right here:

Backward loop

(Scroll down the page to where it says “Single Cast-On” which is another name for this method.)

I like a knitted cast-on for that situation. There’s a video here.

Amy’s doing it from scratch, but once she gets the first stitch on there, it would be the same thing…just insert your needle between the last and next to the last stitch and go from there.

Thank you both. I’ll do that right now and see how it goes. That’s a big help. I didn’t expect an answer so quickly.
Blessings on you both.

I wouldn’t use the backward loop co for more than a couple stitches, you end up with extra yarn when you knit back across them. Use the knit or cable COs, they’re a much better edge.