Top down in the round is about the fastest way to knit a sweater. I don’t know why sweater you’re talking about. Is it a raglan with sleeves? Just follow the instructions, it works. You cast on at the neckline, but right from the beginning they will (I’m guessing) have you place stitch markers, dividing the circle into 4 or 5 sections. Between two of these stitch markers are the stitches that will make up the back. On either side of the back will be the sections that will make up the sleeves, on the other side of the sleeves will be the front piece(s). You don’t knit the entire sleeve this way. You continue increasing on each side of the stitch markers until the stitches in the back section are enough to go across your back. Then you fold the sleeve sections over and put those stitches on a holder, and by this point they just make little sleeves. Then you will continue to knit the body and once you are done with the body, you’ll go back and knit the sleeves.
Trust us, it works. Just follow the pattern.