casting on to existing pattern

[soosiegram]

My pattern tell me to cast on to existing piece to shape the armhole.It says cast on 34 sts at beg of row, pattern to end of row.I have no idea how to cast on to something that already exists. Can someone help:

[suzeeq]

Use the knitted cast on. Turn to begin a row, knit into the first st, but leave it on the needle and put the new stitch on it as well. Repeat. If you look at the video it starts with a slipknot, but th