How do you cast on the middle of a piece?

Directions say: “On the following row, purl 60, cast on 30 sts over those bound-off, purl remaining 20 sts.”.

No idea how to cast on in the middle of the row. Any ideas?

You can do a cable cast on or a knitted cast on. Either will work well and both are demonstrated in videos on the Cast On page under the Free Videos tab at the top of the page. You’ll basically switch the needle from your right hand to your left for the cast on, then after the sts are cast on, switch the needle back to the right hand and continue with the “purl remaining 20sts.”