If you want to be a really good GM who seems like they plot things waaaaay in advance and are really good at foreshadowing, the best thing you can do is re-watch your own games (or take really good notes) and keep pulling from what's already happened.
It's really hard to intentionally foreshadow something in a TTRPG because you never know what players will do or how things will change, but it's really easy to SEEM like you foreshadowed something by just pulling on a thread that already happened.
(This, btw, is what the writers of Adventure Time did. A lot of stuff from early seasons that comes back wasn't planned, it's just that the writers were really good about looking back at what they had already done and developing it more, or building off a line that seemed like a throwaway at the time.)















