Use a skill in ChatGPT
Upload the zipped skill once and ChatGPT uses it automatically when a request matches. Worth checking your plan first, because this one lives on workspace plans.
Skills live under Plugins in the sidebar rather than in settings. If you cannot see a Skills tab at all, skip to the last step.
Open Plugins, then Skills
Skills live under Plugins in the sidebar rather than in settings. If you cannot see a Skills tab at all, skip to the last step.
The one that catches people out. Skills do not follow you between surfaces. Add one on desktop and you will still need to add it again on web or mobile.
If your plan does not have skills yet
Both of these get you most of the way there with the same file.
Custom GPT
Create a GPT, open the Configure tab, paste the skill instructions into Instructions and upload the file under Knowledge.
Project
Make a project, drop the file in, then open project settings and paste the instructions there. Projects are on every plan, including free.