I thought I'd share how I shortened a bias skirt from the elasticated waist. I didn't remove the existing elastic as it would have taken too long to pick out all the stitches.
First I measured down to the new required waistline. The safety pins are holding the two layers together but are not strictly necessary.
Then I made a second marked line two elastic widths above the first.
I trimmed away the excess along this second marked line, then put the outer layer inside the lining, right sides together.
I measured the amount of elastic required for the waistline and joined it into a circle, then pinned it to the newly cut edge from the lining side, matching the half, quarter and eighth marks. Using my overlocker with the blade disengaged, I sewed the elastic to the skirt, stretching the elastic to fit the fabric.
This is what the finished overlooking looks like. You could use a zigzag or three step zigzag if you don't have an overlocker.
I then turned the outer layer over the top of the elastic and into the lining, wrapping the elastic edge with the outer fabric. I then used a zigzag stitch to anchor all the layers together, stretching the elastic to match the fabric as I sewed.
I then turned the skirt right sides out, and steamed the stretched out elastic to bring it back to the correct length.