@J0llyh0e personally I try myself, plan the main ideas, plot twists and then space it out and find a flow. say, there's two events in the books. I space it out with chapters trying to link them both. Also, idea wise, you may take your main idea as simple as it is and try to add to it to make it more interesting. Like a conflict, a disease, a personality type to just make the end goal difficult to obtain and add vibes yk? Plus it makes things more interesting for their relationship ;)
Then again, I'm not the best either but this is what I thought of. Hope it helps