Most underwear have the same steps!
Here is a How To Sew Underwear tutorial.
This focuses on using only a Serger and Coverstitch, but those machines are not necessary!
These Underwear streams go in to sewing them the most stretchy way possible. The second one becomes an experiment.
These videos go great with my How to Sew Picot Elastic and How to Sew Fold Over Elastic videos!
Here's a summary of the best way to sew elastic for maximum stretch.