Start with the T shirt you want to cut into the vest, and scissors. Any type of shirt is good. Keep in mind that the fabric will curl/roll slightly along the cut parts since its a no-sew project. :)

First step! Cut out the neckline, and two or three inches beyond that to give a loose hang-on-the-shoulders type of fit/feel.

Cut straight down the middle.

Now focus on the sleeves. Cut straight up to the sewed line (but don't cut over it!)

Then cut around the sleeve following the sewed line until it is completely off. Repeat for the other sleeve, and it should look like this picture. It should slightly resemble a tank top sleeve or whatever you call that.

OPTIONAL STEP!
See the square edge a few inches to the right of the sleeve holes? They can pop out when you are wearing your vest to give a very square-y look. If you don't like that, you can grab your scissors and start from the bottom, and cut up and make a gentle slope to the top, so that the bottom flares out but the top is sloped and is not square.

Ta-dah! here is my result. Hope my 'How-To' was understandable :)

The back! Yay!