1. Fuse knit interfacing tape to the entire length of your hem, on the wrong side of the fabric.
2. Serge the raw edge of the hem. This is really going to help with getting a perfect finish!
3. Fold up your hem and press.
4. At the side seam junctions, clip into the serging just up to the needle thread. Push the seam allowance in opposite directions - now, when you fold the hem back up there will only be two layers of fabric at the junction rather than three, and it'll be much easier to sew over.
5. Use wonder tape to temporarily baste down the hem. First you stick the tape to the fabric, the peel off the backing tape and press the hem down.
6. Place the garment right side up on your coverstitch machine, and sew from the right side. Start on a piece of scrap fabric, and then "run on" to the hem (you can cut the scrap fabric off later).You should be stitching directly on top of the serged edge on the other side - you should be able to feel it with your fingers as you feed the fabric through the machine. Optionally, you can first hand baste through the serged edge to give you a guide to follow when you're on the right side, but I find that feeling the serging underneath works well.
Use a tapestry needle to feed through the serger tails back into the stitching to finish.
And you're done!