As someone who is kind of in love with foreshadowing, this is my very short advice:
Don’t.
And by that, I mean write a story that doesn’t rely on foreshadowing. You need a story to stand on it’s own after chapter one, and every chapter needs to be engaging and moving forward. This means that in essence, you need two plots: Immediate and the Long Game.
The immediate plot is what makes the character move forward now. It’s the reason for going from point A to point B, and get involved with person C or conflict D. This is what the player will engage and have fun with.
The Long Game is just there. Clues are planted, weird shit happens, but it’s all in the background. Towards the end, this will start adding up and astute players will spot it before others. The trick is weaving this into the story, and accept the fact that a fair subset of players won’t remember or add things up. So the rest of the book needs to work for them regardless.
How to do this is very hard to describe without knowing the details, but here are a few things I learned while writing Fallen Hero (who has a few… hmm… layers).
1: Be honest in the text. Always write the truth, if you do it right, people won’t connect the dots anyway, but once things are revealed everything will make sense.
2: Don’t obfuscate, just don’t explain. Is a character weird about a thing? Write that, but don’t linger on it. Remember what the character knows, and write it from their perspective, not the readers.
3: A main character can know things that the reader doesn’t. Sometimes that’s a fun mystery too.
4: Pay attention to your playtesters. If they feel confused, or if things came out of the blue, you need to plant more clues.
5: Never underestimate your readers. Start with no handholding and give them the benefit of the doubt, it’s easier to add more info afterwards.
6: Some players will remember lore better than you. Other players won’t remember anything. You need to cater to them both.
7: Make the book enjoyable regardless of whether the player adds up the clues or not.
Probably more, but dinner is ready so… I might get back to this topic, it’s my favorite kind of story.