I think that if you just start decreasing around the center, you will end up with a v-neck. If you want a rounded neckline you will likely have to cast off the center stitches and then decrease gradually up to the shoulders. It might be a good idea to look at other patterns that have a similar neckline to get an idea of what's usually done.
Brave you for knitting a sweater without a pattern!!!! :muah: