Mainly a new knitter.  Working on Pattern by LionBrand #70358B "Baby Pancho".  I figured out pattern until part which state Shape Neck.  I had to join a new ball of yarn, bind off 16 sts., k21 and continue to work each side at same time with separate balls of yarn, keeping 3 sts at each side edge in Garter st.  Work even for 3 rows.  I did that. 
Then it states: Next Row (RS-cast on back neck) : K19, cut second ball of yarn and continue with one ball to join each side, cast on  20 sts., K19 sts .  This is what I don't understand, how do I cast on? and onto which needle,  I would appreciate any ones help.