These are often called One-Seam Pants. There are 2 basic approaches. In either case, you will need to cut two pieces and then place them right-sides-together and stitch as directed.
1. Buy a pattern and follow the instructions.
If you can look at these pictures and see how it works, you can do it. Measure the circumferences of waist, hips and legs. Determine the desired length. The top edge can be folded over to create a channel for elastic insertion.
Also, if it turns out terrible the first time, you will learn from your mistakes and laugh a lot. This is a great activity when you or your spouse are bored. I would imagine you can do this with children, too.
The spouse may not understand the nature of the fun, but if you persist, I promise you a good time.
This was a public service announcement for all you lovely readers who have read that post, sent me questions and, well, just admired those towel pants!