Sweater neck question

I am making my dd a sweater. Trying to get it done before spring gets here. Anyway, I need to do the neck and I am changing the pattern… I want to do a plain rounded neckline, like on a t-shirt, so I won’t have to do to much else to it. I guess I need to split the stitches in half and work each side seperately. But how do I do the decreases?