A clean finish on a hem that is not straight is tough. The curve tends to pucker a hem. To prevent this you can staystitch while applying your finger to the back of the foot thus applying resistance to the fabric from coming out freely. This is also known as staystitch plus. This eases the fabric as noted by these tiny accordion type looking pleats:
If your fabric is very convex then you may want to run a basting stitch and pull up the fabric to gather it even more in order to fold it in neatly.
Now turn your raw edge up right at the stay stitching line, using this as your guideline for folding and pressing, press under once more so you have a nice clean edge:
Now I use my overlocking foot to do hems a lot of the time. This is my cheat method. There is a knife like projection in the middle of this foot. This is your guide that you place the fold next to as you sew you get a perfectly straight and even hem!
How easy is that?