(Click on the photo to enlarge and see the steps)
So the first thing you need to do is lay out your Tshirt. Depending on the design on the front (if there is one or not) these steps may be different. For mine though, I measured from A to B and made sure that it was the same size as the like from C to D. You want them to be around the same size.

Now cut from A to be through both sides of the T-shirt. Cut lines C to D and E to F only on the front side of the T-shirt and around the back of the neck.

From cutting out the square you should have a piece that looks like this. I'm saving mine as a bib for a baby but you can do whatever you want to with it :)

I'm sorry I don't have pictures for this step, but hopefully you can follow along without me drawing it out for you. If you have any problems, feel free to ask.

Now you want to take a seam ripper or scissors and cut the seams from A to A' and B to B'. I kept my seams and only tore out the original sewing because I needed more fabric, but you can do whatever works for your project.

Again, I'm not the best at illustrating, but this step is easy!
Put right sides of A-B of one half of the bottom of the T-shirt to line C-D. Sew along that line. Do the same for the other piece of the bottom of the shirt and line E-F.

And Voila! You're done :) (it should look like this)

I did a top stitch across these parts and around the back of the neck, but it would look fine either way.