Sweater sleeve question

Hello! I love knitting! This is my second project, and I’m stuck. I am working on a basic baby sweater. The instructions are as follows:

(RS) Work first sleeve (left). Join small ball of yarn to the right sleeve and work across. Work 2" on sleeves then start dec on each side of sleeves thusly: K1, k2tog, k across to last 3 sts, sl 1, k1, psso, k1

I am first stuck with joining a small ball of yarn to the right sleeve - how do you do this?

Thanks!

Just join as if you ran out of yarn in the middle of knitting…