Seemless sleeves

So my seemless sweater is complete except for the sleeves… the pattern says to place my sts back onto 2 circ ndls and pick up four more from the underarm. Then what? Where does the yarn i’m supposed to be knitting with come from? Was i not supposed to tie-off after BO? This is my first sweater EVER and i’m confused… Please help me!

You have to take a new yarn end - leave a tail to weave in and just start knitting with it.