How to cast on extra stitches?

I’m knitting some little shorts for my baby and I’m knitting in the round. When I split for the legs the pattern says to cast on 2 stitches and then join the round and knit in the round for x number of inches. How do I cast on an already started project? Is there a video on this site on how to do it? or can you explain it to me? Thank you! I’m stuck till I figure this out.

When I have to do this, I use a backward loop cast on. If you don’t know how to do it, there is a video for it. Better to see it than to explain.

I always use the knitting-on method when a pattern calls for casting on additional stitches…video here.