Pants are the made the same way, just make the legs longer.
I can make a pair of these in about fifteen minutes. I make a lot of similar things, like ultralight windpants, fleece pants to wear under them, bib foulie rainpants, etc. It's amazing how fast it is to make something like this, which costs big dollahs in outdoorsy stores.
Fine photos by Star
Shove one pantleg sleeve into the other and flatten as seen here.
This will be your pattern.
Step 2: Fold Over the Fabric
Lay it flat on the ground and fold it over as seen here so you can cut out both pieces of cloth at the same time.
You won't accidentally make two left legs this way.
Step 3: Trace the "master shorts"
It doesn't matter how dark/contrasty your marker lines are, none of them will be visible when you're done sewing.
Step 4: Cut Cloth
If you want to copy a pair of shorts exactly you should draw a "sewing line" and a "cutting line" outside that. The "seam allowance" is about a half inch for seams and hems, and about an inch for the drawstring at the top.
If you're making nylon windbreaker pants, you might want to melt the edges with a lighter or "heisschneider" to keep the cloth from unraveling. Plan ahead though, those melted edges can be scratchy.
Step 5: Drawstring
Use what you can find.
I measured this string by wrapping it around my waist and leaving extra to tie with. It was too short.
I should have compared it to the waist of the shorts, which is bigger than my waist so I can get into it.
If you prefer an elastic waist, get some elastic strip when you're at the fabric store. The rest of the steps will be prettty much the same. Remember to stretch it flat while sewing so it can bunch up a bit afterward.
Step 6: Sew the Flies
This is a "peel seam", very straightforward, needle goes through two layers of cloth.
If you're worried about the inside edges of cloth unravelling, you can do a zigzag stitch along that edge. That's kind of what a "serger" machine does. If you look inside the clothes you're wearing, you'll see seams like that.
Step 7: Sew the Inseam
Open it up as show in the 2nd photo and lay it flat again looking like a pair of pants.
You're ready to sew the inseam. Do it just like the previous step.
Step 8: Stab some Holes and Sew the Drawstring Top
There are fancier ways of doing it but this is good enough.
Poke the two ends of the drawstring through the holes and fold the cloth around the drawstring. Fold again so the the cut end edge of the cloth isn't exposed. Sew the hem. Don't sew through the drawstring. Actually you might want to tack the drawstring in one place in the back so it doesn't come out in the washing machine.
You can sew the pants first and install the drawstring later, but it's easier to sew the cloth around it.
Step 9: Hem the Cuffs
Probably you'll need to cut the cuffs even, they hardly ever turn out straight from cutting flat.
Speaking of drawstrings, you can do drawstring cuffs if you want.
Then you can close them off and use the pants for a bag.
I've added drawstrings to cuffs to pull my feet in entirely and close them to keep bugs out.