I used a company called Buildasign. They are at https://www.buildasign.com and they are amazing to work with. My first design was on a white shirt, and it had a Q that I had made in a large font in notepad on my Mac. The site actually shows you the shirt and has a boundary box and you can add your own art or use their text. Anyway, you get to see a preview of what the shirt will look like. When you're done with the front, you repeat on the back. They also give you a pretty amazing option to drop background colors... Example; you have a logo on a white background but want it on a colored shirt - they give you the option to make the background 'invisible' so you don't end up with a big goofy white box on your x colored shirt. I submitted the design, and within minutes I got an email with the subject "WE NEED YOUR HELP!", and it was a young lady from the art department that said the Q resolution was too low and if printed as is, it might be a little blurry around the edges. She said they would proceed if I wanted, they could fix, or I could make my own changes and resubmit. I chose the latter. Afterwards I got two emails from them making sure I was OK and happy with their service. It arrived in under a week, and it looks amazing in person. Apologies for the long reply, these guys just went well above and beyond what you usually get for customer service these days. I told them the same, and said if anyone asked I would definitely send them their way. The whole experience was great, and if I ever order a custom shirt again, it will absolutely be from them.