Using 2" elastic, I wrapped it around my waist where I wanted the skirt top to sit, stretching the elastic a wee-bit tight. I need to make sure my skirts aren't going anywhere - I have a toddler that likes to pull at them! Then I added 1" for a seam allowance.
After sewing the two ends of the elastic together, I marked the elastic into four equal sections, using pins.
I also marked the skirt into four equal sections (the side seams are the first two!).
Putting the elastic and skirt right sides together, matching the pins on the elastic to the pins on the skirt, I serged along the top, stretching the elastic to fit the skirt.
Then pressed this serged seam down, towards the skirt.