How do you go about it as a potential mill customer?

[Kitfoxdave]

There are several companies that have a path for custom tartans. USA kilts and St Kilda Kilts are two that work with more than one mill. Typical minimum order is about 10 meters. They both have pricing information online, as well as tartan designer software tools.

You can have custom dye colors done, but there is an upcharge for that.

Youtube has some USA kilts videos of the process of custom tartan. These will hopefully help you.

I have been collecting embroidery thread to determine the final colors I want for my custom tartan, and plan to send them along with my request when I get to that point.

There is an individual weaver on the west coast, that does small custom weaving jobs like making a sample of a sett... it has been a while since I looked into it last year.... but a Google search for Custom weaver in USA will likely bring up her details.

Good luck with your project!