I usually began designing t-shirts by sketching ideas I have had, then taking these ideas to Illustrator to flesh them out.
Once I reached a finished design I would export them as hi-res vector files to be sent to a screen printing company I used based in Bristol.
To add a touch of individuality, I devised a way for each t-shirt to have a unique numbered label sewn onto the front. For a small micro brand this was unheard of, as usually something like this could only be achieved with economies of scale, but I would only create 100 pieces of each design.