Have you considered turning a description of the commissioning process into wording for a custom listing? It could even be so clear as to use the heading - Custom Listing: let me turn your button down shirt into an heirloom piece.
At the start of the listing make it very clear that you need a clean shirt with a flat, unembellished 15 ins square surface area. Explain all the basics, that they need to mail it to you, that you'll let them know when their shirt has arrived, what you'll be doing to the shirt, whether you'll use all of it, a little bit about your process (without giving any secrets away), any opportunities for customer input on how the finished item will look and an up to date estimate of how long the work will take, and when they should expect to receive the finished product. That way everyone knows where they stand, and it could save you a few back and forwards with convos or emails, too.
Wishing you every success - I have a couple of things I've made for myself from favourite clothes I couldn't bear to throw away, and I smile whenver I see them.