Romantic love writing comes naturally for some, harder for others. Let's start by breaking down the barriers that separate the naturals from the strugglers.
Close your eyes and think about that one special person in your life. Maybe they were your first love. Maybe they're your spouse.
Try to remember all those wonderful things that attracted you to them. Stir up those feelings inside. Now apply that emotion to your characters.
Be careful, though. Just because you've conjured up those wonderful memories doesn't mean you're ready to shower your readers in rose petals and chocolate hearts.
First off, we need to examine the wrong way to write about love:
You've probably read books where you didn't fully understand how the character really felt about someone. Why didn't you understand? Artificial writing. You were told how they felt. You didn't feel it.
One of the very first items you should mark on your "not-to-do" list is using clichés. You know the type:
Did you feel anything? Besides nausea.
Does this describe how you've ever felt about someone? There's no depth in the statement. Similes don't offer any real substance to a powerful emotion like love.