Ruined a pair of old knit shorts I used to clean and paint around the house. Almost went out to Kmart or somewhere to replace them but decided to use a Benetton USA super oversize T-shirt that I received as a gift for a purchase over 15 years ago. I hardly ever wore it, way too big, no time to sew before now. The following is how I used a large pajama bottom pattern to make them into a pair of much needed slouch and sleep shorts. Very comfy!

Step 1: Lay Out the Pattern.

Once you find a large enough T-shirt, this one was a long XX large, just lay out the 2 pajama bottom or short pattern bottoms that allow for an elastic waistline. I had to overlap some of the pattern at the side seams, but not by much and the pattern was large to begin with. I just knew to be sure to make the seams a skant 1/2" instead of the usual 5/8".

Step 2: Cut Out the Pattern and Stitch.

This is a very quick project and once you cut the pattern out, you will stitch up the front and back of the pieces first, then you will stitch the sides, joining the front and back panels, as well as the crotch area. Clip the curves on the front and the back crotch area after sewing. Since I was working with cotten knit, I used a stretch stitch to allow for more give in the seat area. It is a good idea to double stitch the seams if your machine does not have a stretch or knit stitch for extra reinforcement. You can see that I cut into the neckline of the T-shirt as part of the pants, but that this ribbing will be on an inside seam.

Step 3: Sew the Waistband and Insert the Elastic.

You can finish the seams as needed. Some knit locks tight and Po' Girl thinks it is a waste of thread to finish a seam no one will see anyway. I stitched the seams down in the areas at the very top of the waist before turning them under to sew for the elastic. (I apologize that these pictures are a bit hard to see details.) This way when you turn the fabric under for the elastic casing, the elastic will not get stopped by your seam fabric. You only need about a 1" casing since you will be using lingerie elastic which is thin, about 1/4"-1/2" and very stretchy. You will regret using the heavy and wide pants elastic when you try to sleep in them. Thread the elastic through, leading with a safety pin on the end, push and pull til you come back out the same hole you left when sewing the casing. Join the ends of the elastic, overlapping and sew securely. Did not bother finishing my pant leg hems either. You can if you wish. Because of cutting out from the T-shirt, the back leg panels were slightly longer and since this was a really fast and lazy project, I did not bother to trim them up for finishing.
In fact, this Instructable may have took longer to post then making these shorts!
A tip is to be sure of where the waistline falls on any pattern you are using. Many of todays patterns have waistlines that fall below people's natural waistline and they can be uncomfortable depending on how you prefer to wear your clothing. You can always add additional inches to the crotch by cutting the pattern across the alteration area and add additional paper if you have the available fabric.