First, I put the skirt on, and I measured where the straps would go.
I cut two pieces of elastic to this length. Then, I used chalk to mark where the straps would be going. I put the chalk marks on the inside of the skirt, so they can't be seen when I'm wearing it. Next, I flipped the skirt inside-out so I could sew the straps onto the inside.
Next, I used fabric glue to attach the elastic straps to the chalk markings.
Once that was dry, I put the skirt back on with the straps over my shoulders. It still looked a little bare, so I decided to add one more strap.
I measured around my waist, about two inches above the top of the skirt, and I used chalk to mark the insides of the straps where I wanted this new strap to go.
I cut another piece of elastic to this length, and I used fabric glue to attach it to the inner sides of the shoulder straps.
Here is what the finished project looks like:
