Place shirt flat on a flat surface (u can use a t-shirt too I just chose a long sleeve because I had an old one I never wore great time for a recon ;)
Next, take scissors and cut off the sleeves, I find it looks the best if you go right under the seem line of the pit.
Next cut off the collar of the shirt.
Optional** You can choose a different neckline if you want to be creative with it, the one in my picture however is just cut around the seems of the collar
Now cut directly down the middle of the front of the shirt
Now try it on, if there are any hanging pieces don't worry it does this to some looser fitting shirts, just snip them off
*optional: you can wear it like this as a cute vest or take the bottom hanging parts of the vest and tie them together to make a top like the one above and your done! :) enjoy