Layers Magazine has an excellent tutorial by Jeff Finley, on how to create a t-shirt design with Adobe Photoshop and Illustrator in vector graphics.
We particularly like the section about preparing your artwork for screen printers:
“Now that the design is created, what do you do with it? In most cases as a designer, you will be sending the proper files to a screenprinter. All screenprinters are different and may require different file formats. Some have prepress people that do all the color separations and can work with almost anything you send them, as long as itâ€™s high enough resolution to keep the detail on the shirt once itâ€™s printed.
So here are some tips for preparing your file for your screenprinter:
1.) Choose Pantone Colors â€“ Some printers can match colors, but if color accuracy is important to you, you should choose the exact pantone colors that you want your design printed with.
2.) Prepare your design at the exact size it will be on the shirt. If your design is going to be a full chest print, then it should be around 11 inches wide. Set up Illustrator to the correct dimensions and make sure your design is that big.”
We agree that it is important that you set the scale and provide colour refs as this ensures your design prints how you want it to look. Indigo has some artwork guidelines online if you still feel confused.