Here's how I did it. My old pair of jeans
Scissors. Cut along the stitches, don't stress yourself out about this part. I wanted for my skirt to be knee length, but you can make yours shorter if you want to.
Since prewashed jeans are still in fashion, I chose to use the end parts of the pants inside-out. Pin it just under the fly before sewing it. And do the same for the back part, too.
I had some gores from another pair of jeans and I used them just to spice it a little bit.
Voila!