How can I build a custom GPT trained on my own tax planning strategies to speed up producing tax plans for new clients?

One member kept a roughly 100-page 'master library' Word document built over 20 years containing their own tax planning strategies, recommendations, and preferred wording. Previously, building a tax plan for a new client meant spending a full week pulling relevant material from that document. They loaded the master library, along with other files, into a custom GPT so its primary source of content is their own written material (it will also pull more current information from the web if asked). On a recent prospect call, they fed intake-form background info to the custom GPT and it produced a solid tax planning report in about 10 minutes instead of a week, letting them spend the call time walking the prospect through recommendations instead of drafting.