When a pattern asks for a narrow hem, they usually mean a 1/2 inch hem folded in half that results in a 1/4inch hem/fold you have to top-stitch to keep it in place. That sounds easy enough and yet when you are at it things get a little bit dicey when playing with a steam iron and such tiny folds...
Here is the technique I use every single time to produce small clean hems:
With your 1/4" quilting foot, start by 'drawing' a line in stitches a 1/4" away for the edge of the fabric
Use this guide 'line' to fold the fabric over 1/4", I have an hem plate from Dritz I like for this step.
Now fold the hem over itself once more to enclose it, you will have you stitches showing on the fold.
It is now time to top-stitch your hem. Pin it well first, then flip it over so that the cleanest side of the stitches is visible on the right side of your project. I use the #5 foot on the Bernina Aurora, you could use your own blind hem foot or an edge-joining foot. I also move my needle all the way to the left.
My project was a baby pouch sling and I decided to make two rows of top-stitching to make it extra-strong.
Here again I used the 'blade' in the middle of the foot to guide me. Keeping my needle all the way to the left, I made the blade follow the previous stitches.