1. Torrent the shows you watch, this way you'll be able to quickly and easily take high quality screenshots.
2. Take screenshots constantly, whenever something stands out to you. It could be a funny face, a funny line, some crazy shit in the background, who knows. Name it appropriately or it'll be lost forever in a pile of random images.
3. Post your screenshots whenever you think they're relevant.
That's enough for a bare minimum of shitposting contribution, but for real pros you need to take the next step:
4. Download GIMP. (It's free!)
5. If something happens and you want to respond, but you don't have an appropriate image, make it yourself! Edit one of your current images!