Once you have drafted your context document, you have two ways to use it.
The first is to paste it at the start of any new conversation. Copy the document, open a new chat, paste it as your first message, and then follow immediately with your task. This works on any platform, any tier, and gives you full control over when the context is applied. It takes about five seconds and the difference it makes to the output is immediate.
The second option, and the one I recommend for your most frequent tasks, is to save it in your platform settings so it is applied automatically to every conversation without you having to do anything.
In ChatGPT, this is under Settings, then Personalisation. You will see two fields: one for what the tool should know about you, and one for how you want it to respond. Your context document covers both.
My practical advice is to do both: save your context in your platform settings for everyday use, and keep a copy of the document itself so you can paste it into conversations where you want to supplement or override the default, or when you are working on a new platform.