Does anyone know a simple guide to installing/downloading an offline chat GPT AI ?

I have been told all I really need is a solid gaming PC, which I have, and I would like to test out Dungeons n Dragons (I know, I know lol) writing prompts with the bot and work outside the limits of OpenAI. Can anyone help me? You tube videos would work best preferably tbh I am not very tech savy and I find a lot of written guides on things like this to often leave out small parts in between steps that they feel are obvious.
Thank you in advance to anyone who is willing to give me advice! :)