baby/doll sweater

[javede]

So, my mum completed her scarf much faster than I expected(4 days!).
Now she's knitting a doll sweater based on this pattern:
http://www.craftown.com/knitpat18.htm
and of course I have to help her.
My problem is how do you cast on stitches on a piece your working on(here to form sleeves)?
It's quite urgent, cause she will be at this point tomorrow if she continues at this speed!