Pin the pieces you just cut onto the inseams with pins.

Sew the edges together.

This step will take a long time to do by hand, but it is possible. I still recommend using a sewing machine if you can, so the process goes a lot quicker. The stitches will also be more secure.

Be careful when sewing. While doing it by hand, you might poke yourself. When using a machine, keep your fingers away from the needle.

Try on one pant leg. If you don't like it, it's not too late to undo the stitches and either make it wider or slimmer. Otherwise, you're done!