Okay, now you have characters and they have motivations. You've developed a conflict. If you've gotten to there, you probably have some idea what you want to happen in your story. Now, there are plenty of different approaches to this stage of the game, but for now I'll start with one.
Figure out where you want your characters to end up.
Yes, it's great to say, "Okay, so Joanne wants to move to Paris and Luke wants to marry her and stay in Oklahoma". Now you have to say "Joanne and Luke will get together in the end, but they'll move to Paris together."
Many, many authors will do things differently, and what works for them is always great advice. But here's why I like this method.
Now you have to figure out why your characters end up there.
If Luke and Joanne move to Paris together at the end of the book, that means Luke has decided he loves Joanne more than money and is willing to give up the latter for the former. For this to work, Joanne has to learn about Luke's plot and about his eventual decision to abandon the money.
Well, if I were writing this book, the first thing I would tell myself would be, "I want Joanne to be strong and independent. I want her to be the type of woman who wouldn't compromise her future for a boy. So I want Joanne to discover Luke's intentions and dump him for it. But if they're going to get back together, Joanne is going to need honest proof that Luke's changed.
"So, in that case, we're probably going to need a showdown between Luke and his grandfather. Maybe Luke could pack up his stuff and leave? Oh, I kind of like that. Luke gets in a huge fight with his family and announces that he's not going to let their manipulations get in the way of him being with the love of his life. So, in that case, maybe his family tries to set him up with another woman once Joanne dumps him. Oh, that could be good.
"But for maximum dramatic impact, Joanne has to be seriously considering marrying Luke when she finds out about his plans. Maybe he could have proposed and she was thinking about it. Maybe she'd even accepted, and when she finds out she breaks the ring. But why would Joanne, a strong, independent woman, be willing to abandon her dream? She must fall really hard for Luke, but that wouldn't be a good enough reason. Maybe there are financial difficulties? That would be a good external conflict for Joanne. So, she's having some financial difficulty while falling desperately in love with Luke, and when he proposes she sees this as the best outcome for her.
"So then the story would have a recurring theme of love vs. career. Both characters are willing to give up the former for the latter at the beginning (Joanne is willing to leave Luke for Paris, Luke is willing to settle for his high school sweetheart rather than wait for the right person) and later on, both have a moment of choosing love over career. And at the end, the two strike a balance. Maybe, at the end, Joanne won't be able to afford Paris, but she decides to try and make it in New York instead, and she stays with Luke. And maybe Luke finds something he's truly passionate about and pursues that. Yes. Yes, I like that."
Either on paper or on the computer or talking to a friend over the phone, ramble your way through the novel. You'll develop an idea of who your characters are, what challenges their going to face, and where they're going to end up. Once you've figured this out, it's time for the real stuff.
Decide how long you want this novel to be. I'll go with 70,000 words, a decent length for a YA romance. And I'm going to write a chapter a day, on any day I can work. Each chapter will be 2000 words.So that's thirty-five chapters. I'm going to divide the novel into the point-of-view (POV) of both characters. 17.5. Hmm. I'll write 18 chapters each.
Now, decide on the necessary scenes. I think I'll start with graduation, and the novel will take place over the summer. Come up with a basic timeline of what must happen for the story to reach it's conclusion, the bare minimum of things.
And then come up with things that could be fun to write.
You want a scene between Joanne and Luke's grandfather? Okay. You kind of want them to go to a ball? Luke's family can hold an annual one. You want-- hey, how about a love triangle! You'll introduce the new love interest for Joanne halfway through the summer, Luke can get jealous, and he'll realize that she's more than a means toward money for him.
And so on, and so forth, until you've got all the chapters plotted out in a fun, logical way, where the characters' actions are consistent with their character development and you really want to write. You now have a plot, and a plan, and all you have to do is write 2000 words a day for a little over a month. Then, of course, comes revision and whatnot, but for now, let's just focus on the first draft, yeah?