Well, the procedure for putting such a thing on a shirt would be to scan the image, set the image up as you would like it to appear, print it in mirror image unless you are printing on opaque transfer, and apply it to your shirt.
But, I hate to rain on your parade, you can get away with wearing it your own self, but selling such a thing is a big RISK due to copyright infringement.
Could be that he's trying to print his own book cover on a t-shirt. Many authors look for different ways to market their books.
It could be done via a heat press, or if you have the cover design as a high resolution graphic file, you could open a store at cafepress, zazzle, spreadshirt, etc and upload the design there. It's free to upload a design, and you can have your book cover on any number of products. They handle the printing for you.
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