heh, do you want to enter the design on a napkin? Sorry but the technical stuff is as important as the idea. Otherwise you would get a blob of mud on your t-shirt. Quality is important.
There are step-by-step how-tos on the cafepress links I posted if you want to format your files. If you have any questions, ask me. Happy to help.
Let me be more direct in my point. This is too techy for the common person who isnt involved in graphic design or photoshop. It seems easy to you because you are a computer artist, but the common user has been eliminated by the techy portion of it. Hence the great interest in the beginning of this, yet only two people have entered in a week. How about a someone to mail your design to that will scan it and set it up? that way you dont eliminte designs from the guy who doesnt have photoshop? Or a scanner? Or is just a computer idiot like me (wads up napkin and throws it away)?