The technique I'm going to show you in this post is for altering the legs on pants that are already constructed. I'll cover altering pants patterns in another post. Also, this works best with pants that already fit you properly in the waist and hips.
You'll want to make sure you are using your sewing machine and not a serger for this. At least until you get the fit exactly the way you want.
The easiest and quickest way is to use a pair of pants that already fit the way you want them to as a guide. Lay the properly fitting pants on top of the pair you are slimming, lining up the crotch and outside leg seams, and trace the leg of the skinny pants.
You’ll want to mark your desired seam on the inside of the leg, starting at about mid-thigh. Your alterations will be less noticeable in this location than if you sew the outside of the leg. Once both legs are marked, you’re ready to sew.
Make sure you are using the appropriate needle and thread in your machine for the type of material you are sewing on. (Denim usually requires a heavyweight needle.) If you are unsure if you’ll still be able to fit into the pants with the newly slim leg, you can sew first with a longer stitch length. This will make the stitches easier to remove if needed.
After sewing, try the pants on. You should still be able to walk around easily and bend your legs. If you can’t, you’ll need to go back and make the legs a little less slim. Or maybe they still aren’t quite slim enough. Once you are satisfied with the slimness in the legs, it’s a good idea to wear the pants at least once before making the alteration permanent by trimming away the excess fabric in the legs.
When you’re ready to trim, make sure you leave at least ½″-¾” of the seam allowance. This will give you enough to finish the seams. You can finish the seams however you like, but I think pinking shears are the best option, especially for jeans. The fabric is just a little too thick for most home sergers to serge without a struggle.