Step 1: Gather Your Supplies
To make sure you get the right size, just make sure you have a pair of pants that fit the intended pajama wearer. If your pajama wearer is far away, then find out the right size and borrow some pants from someone who is that size or carry yourself down to the Salvation Army and part with a couple of your increasingly precious dollars for a busted-up pair you can use as a pattern. This is a DIY tutorial and I figure anyone smart enough to sew up some pajama pants is also smart enough to find some in the right size but you never know.
Fabric - woven cotton or flannel or jersey or whatever floats your boat. The exact amount you need will depend on how big the pants are and whether the print is directional or not. If you are an experienced sewer, you know what that means and you can skip the next bit or you can read it and think, "Well, duh." A directional print is one that has a right-side up. Like words or Christmas ornaments or something else that is meant to hang only one way. If you're making adult pants and using a directional fabric that's a standard 44" wide, you will need almost twice as much fabric because you can't save fabric by flipping the pattern around and cutting one leg upside down. If you don't buy extra fabric and move the pattern down along the same fold to cut the second leg, your husband's cheesy Christmas pajamas will be even more cheesy because the aforementioned Christmas ornaments will be hanging down properly on one leg and, in total defiance of gravity and common sense, hanging UP on the other leg. (Sorry, dear.) To use the smallest amount of fabric possible on your "lounge" pants, buy a cute print that looks the same turned any direction.
You'll also need thread to match, pins, paper to make the pattern, a pencil or pen, scissors, a safety pin, and enough elastic to go around the waist of the wearer plus about five inches. What kind of elastic isn't all that important but it should be about 1 / 2 inch wide.