Every so often I send a partner a message like "ur cute btw" or "did you know I like you?" Something small or silly like that. Also saying thank you when they do something for you. Also just occasionally asking to spend time with them "just cause I think you're neat".

Remembering what they like/dislike, what stuff they are allergic to, if they need accommodations for a disability, learning what to do/not do. Remembering what they've told you but also not extrapolating or assuming beyond that.

As others have said, romance is very contextual so asking and listening and caring are crucial. And just wanting to be around them and inviting them to be around you if they want that.