The t-shirt is customized on the front only.
The QR-code must be very visible and easily scanned, there must be a nice illustration somewhere.
You must display the custom sentence too.
The Twitter username must also be displayed somewhere.
Don't mention mytwittshirt on it, or very discreetly if you can.
You can freely choose the color of the background, but there must be enough contrast for the QR code to work.(ask us if you have any question about that)
We expect to get the high resolution image for the front of the t-shirt, as well as a representation of how it would look like on a real piece of clothes (there are mockup templates here in case you need one:
For the rest, be creative ;-), we'll be there to answer all your questions !!
FYI the Twitter font is Arista.