Sweater Pattern help!

I’m making my first sweater in a cotton blend using this - Mesilla - pattern.

Under the section “Separate Sleeves from Body”, it says this -
CO 3 sts at beg of next 2 rows.

I have 2 questions -

  1. How do you cast on sts in a partially knitted item? I’ve tried to imagine it, but can’t visualize :??

  2. Also, does it mean that I have to continue knitting those newly cast on sts as well as the old ones in st st for 2 rows?

Appreciate your help!

You would use a backwards loop, knitted or cable cast on for those 2 stitches. Look at castons in Basic Techniques here for a video demo.

Yes, knit those stitches into your pattern; if it’s st st, work in st st.