Step 1: Materials
You will also need a sewing machine or needle and thread, I like using a sewing machine though.
And extra material is optional.
Lay your shirt out flat to start.
Step 2: Cutting
Note that the amount that you take off the arms will determine how large the seam at the crotch will be. I did not cut enough on this one. You may want to measure.
Then cut the shirt in half down the middle. I folded mine in half to make sure it came out just right.
Step 3: Unfold and Sew
Step 4: Turn into Pants
Step 5: Sew the Outer Seams
Step 6: Add a waist band
This is just some fabric from an old project.
What I did was fold it over itself. Remember that you can always just hem the edges though, and finish the project without this part.