Ok. Now, turn your existing hem piece inside out. Pull it over the bottom of the skirt, lining up the unfinished edges and the side seams. (The lavender piece above is the hem piece.) Once it's all lined up, pin it in place. Now use a straight stitch to sew around the whole thing.
Flip the hem piece down, so that the skirt is lying flat and the hem piece is now right side out. You may want to iron it at this point, but honestly, I didn't. All you have to do is topstitch around it. Stick close to both the line you already stitched AND the stitches from the existing hem that may be visible.
See? This is the bottom flipped up so you can see where I originally sewed the hem part to the skirt. The black stitching is the straight stitch AND the top stitch from the underside.
So. Let's recap.
The 4 steps to shortening a skirt (that anyone can do) are:
1. Measure and mark.
2. Cut, cut.
Here's a close-up of the top stitching. You can see I tried to stay really close to the stitches that were already on the hem. Looking close, even the blue thread is visible, but you really cannot tell unless you're staring at it from a few inches away. And if anyone's face is just a few inches from your hem line, well, that's just weird.
And THIS? This is why I should get someone to model for me. And why I should NEVER try to look "serious" or give a "knowing smile." Just thought I'd share yet another fail, since I seem to be on that kick lately. Ha!