I like using pins to make a pattern. You don't have to destroy your old garment in the process and you don't need any special tools. Get an old cardboard box, break it up and fold it in half so that there are 2 layers of cardboard for you to work on. Grab some scrap paper or pattern paper and put in on the cardboard. Place your item, underpants in this case, and pin around each section. There were 3 sections total for the underpants (I love that word), front, back, and crotch. After you've pinned the section, remove the pins and garment and then cut around the pin holes adding a seam allowance.
Here are my 3 pattern pieces. I pinned each section separately and then cut them out, adding a 1/4 seam allowance. I actually made the pattern pieces bigger than the original because I wanted bigger underpants. Cut out 1 back piece, 1 front piece, and 2 crotch pieces from knit fabric or an old t-shirt.
Layer the front piece between the two crotch pieces, with the crotch pieces right sides in.
Here's another view of the fabric sandwich. Sew across.
Roll up your back piece as pictured above.
Roll up your crotch-front-crotch sandwich as pictured above.
Sandwich your back piece between the two crotch pieces as pictured above, and sew.
Turn your fabric right sides out and it should look like the above picture. As you can see the back and front pieces are connected by the double layer crotch.
Add elastic to the waist and leg edges. I used a stretch lace, but any softer narrow elastic would work.
Here's the finished product, and 2 more pairs made from old t-shirts. I'm going to make at least 3 or 4 more pairs. They were really easy to sew.
Hope you liked it,