Following up on the recent post about Understanding Color Separations for Screen Printing, here are some extra resources and links for those of you ready to dive in to making your own “color seps.”
This subject does get very dense software-wise. However, most screen printers can make separations for you. So you can certainly be a t-shirt designer and never need to make your own color separations.
Color Separations in Illustrator
One of the most straightforward ways to output color separations is with Adobe Illustrator. See Adobe's documentation of color separating in Illustrator.
If you prefer a video tutorial, this one is good. It will show you how to output your separations from Illustrator. Then bring them into Photoshop and generate halftones if you have gradients or color blending.
Spot Color Separations in Photoshop
For simple spot color separations in Photoshop, this tutorial will get you started.
One-Color Halftones in Photoshop
Photoshop for Screen Printing Halftones Tutorial.
Index Color Separations in Photoshop
This tutorial is about index separations. (However, the tutorial doesn't show you how to create a custom color palette. I will see if I can find or make a better one.)
This tutorial goes into much more depth, but you will have to do a bit of reading.