When knitting a sweater and the pattern asks you to cast-on for an arm how do you do that?
Does the pattern not say how many to cast on? Or do you want to know if you're supposed to do a specific cast on style?
Did you knit the front and back already? If so, whatever cast on you used there will be fine for the sleeves.
If you're to co the sleeve sts to the body, you would use the knitted or cable CO at the beginning of a row. Use the first st on your needle in place of the slip knot, then just start putting them on.