I am not a good tutorial writer but hopefully the pics will help you make sense of what I'm talking about. T-shirt pants are super easy to whip out and take almost no time at all. You really don't even need a pattern.
Fold your shirt in half. Mark a line right under the neck. For the crotch (I hate that word) you can almost just follow the seam of the sleeve straight down and then curve it to under the arm.
If you aren't so confident with your crotch drawing skills just use a pair of pj pants that fit and trace the shape with a disappearing pen. The outside edge of your pants will be on the fold. If you want a wider leg pant use the whole width of the shirt. If you want them to fit like the pair you are tracing mark the width when you draw the crotch.
After you cut this is what you should have. Note: Don't forget seam allowances. And if you want to add an elastic waist (I don't have any elastic) add an inch or two at the top to make a casing.
Open up your 2 pieces and put them wrong sides together. Now you are going to sew both of the curved edges of the crotch.
Now take the 2 seams you just sewed and match them up in the middle. Also match from hem to hem along the inside seam. Like this:
Sew your pants along the in seam from hem to hem.
Flip them inside out and attach a waistband. Sorry I didn't take pics of that but it's the same as doing a twirl skirt waist.
Done! An easy way to use the big shirts in your stash.