You have to get the shirt template. For example take this shirt:
http://www.roblox.com/Bread-Tee-item?id=346360945
Now you would subtract the ID by -1 as many times as you need to find the shirt template.
346360945
-1
__________
346360944
Where you would find this template when you search it:
http://www.roblox.com/Bread-Tee-item?id=346360944
Some times you have to try a few times to find it. Good luck!
~Slothzie~