Step 1: Stitch
I don't actually trim anything off. As you can see in the photo, I align my fabric edge with the blade. If you want to trim fabric off at this step, please be sure to add some to the hem when you cut out your pattern.
The second picture is a little wavy, but don't worry, that will get pressed out when you get to step 2!
Step 2: Press
Step 3: Press again
Fold right at the top of the first hem for a perfect narrow hem. It is really easy to be accurate, because the serger stitches provide some body.
Step 4: Stitch!
You could also use a blind-stitch foot (one probably came with your machine) and flip the work over so the thicker fabric folds are on the raised (right) side on that foot. (See photos 3 and 4).
Remember to choose a longer stitch length than for construction, because you're going through more layers here.