Cardigan sweater band problem

I changed a pattern – decided to put a garter stitch band on a cardigan sweater that had a crocheted edge. This is a band going all the way around the neck and down the sweater front. Picked up stitches from the left and right fronts, plus those on holder from the back neck edge. What’s the best way to get the curves right and ensure that the edge doesn’t stand up in back? My first effort turned out really weird. I have garter stitch band on the pockets and hem, so don’t really want to go to a different border stitch.