Add on?

I am working on a child’s “T-Top” sweater and am a little confused by the instructions for the sleeves and yoke. The instructions say ‘Add on 28…’ what does add on mean and how do you do it?

Thank you,

Probably cast on 28 sts for the sleeve. Do a knitted or cable cast on.

Thank you so much! I’ve been agonizing over this for the past two hours! I really appreciate your help.