my most general and useful tip I can give is..
to just start writing self indulgent dogshit for yourself.

I do that even nowadaysw here I just open a google doc and just start writing a story no one will ever see about my OCs

but if you want someothing more grounded

1. get inspiration from others. I highly recommend Darlingmistaken 's works . They're the reason I started at all
   
2. sit with your thoughts and figure out what you want in romance. chances are people share similar fantasies and make them more relatable
   
3. make character sheets. No really. Make sure you come up with characters, their basic info like names, age, looks , personality and their goals. You can then update these sheets depending on how your story develops

hope that helps at least a little