The best advice I ever heard on how to write characters falling in love, is to make each the characters two people who fit each other's deepest needs.
So, I'm assuming that you already know who your character is. What are his strengths? What makes him stand out as a character? What are his weaknesses; his fears, his needs?
Now shape a woman who will supply his needs, and whose needs she can supply.
For example, let's say your character grew up in a broken home. His father walked out on him when he was quite young, promising to come back but never did. His mother disappeared into a bottle. He was mostly raised by an aunt who later died. He learned the lesson that people lie to you and leave you and you can't trust them.
Now you create a woman who is loyal to a fault. Demonstrate this with a situation where she is fiercely loyal to someone who doesn't deserve her loyalty. Because we know his background, we can see why he would fall for her.
Now ask yourself; what is her need? Maybe she has a bunch of shiftless relatives who use her and constantly borrow from her and see her only as a source of income. And they take her for granted. She needs a man who doesn't need anything from her financially, and one who will not take her for granted. She also needs someone to rescue her from her situation.
Once you set up these elements, the romance can take place organically. Because they are such a good fit for each other, it will feel completely natural when they fall in love.
People from dysfunctional backgrounds often make choices which perpetuate their situations. It would make sense that in the past she has fallen in love with men like her relatives. Men who need to be rescued, who are needy. Each time, those men have failed her because she eventually came to realize that they didn't love her, only what she could do for them. Your hero can be different. All he has to do is prove in some way that he is not like the other men, and that he will put her welfare over his own (which is a thing the other men never did)
Experiment with different character traits and combinations until you find one that fits. And then have fun watching the characters write the story themselves :)