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Any tips on preparing the t-shirt design for printing?

I've been looking for some info about this for months but I can't seem to find any thing about this.
What are the technical requirements for screen printing?
Is it better to work with vector or rastra?
How does the printing treat halftones or color that is like with 50% opacity?
Maybe you have some other tips you'ld like to share regarding this topic?
Tutorial links would be nice too.