Read, read. read and then read some more. See what you like in romance, see what you don’t. Find what you think is working and unique, and find all of the overdone cliches you probably want to stay away from.
It all depends on what kind of romance you’re wanting to write but, I would say the most important thing that many romance writers can get wrong is forced chemistry. Know your characters. Know why the couple is drawn to each other and let the readers see that, too.
I can’t tell you how many novels I’ve read from younger romance authors where the couple isn’t together for any other reason than the author wanted it that way.
Chemistry is a vital component to get across to your readers between your desired couple.
Also, this is my personal opinion but for a slow burn romance- even if it’s the sub plot- there needs to be tension. Sexual tension, emotional tension, ect. The best part about reading a romance is the build up to the couple finally admitting their feelings or finally giving in and kissing. The build up of tension is key to a successful, final explosion of passion whenever they decide to give into their feelings for one another. It’s always, without a doubt, my favorite part of romances.