I usually approach romantic subplots from the point of view of ‘what can these characters give each other and what can they teach them’. Both characters should have their own character arcs and development, neither existing for the other, so you need to work out where the relationship fits in in the context of their journeys.
As an example, in my current work, my character A is dealing with issues surrounding identity and belonging, while character H is dealing with issues around regret and redemption. H gives A a shoulder to lean on and the comfort of belonging that A hasn’t had before, while A gives H stability and shows H that everyone can change and that everyone has choices. All in all, this creates a situation where their relationship helps both characters grow in their own directions and as these characters move closer to development, it creates conflict that both of them have to go through.
Other than that, deciding on their dynamics is helpful (do they banter a lot? Are they reserved? Do they bicker or are they the types that blow up every once in a while?), also, the pathway of their relationship development (do they go from close friends to lovers, or from strangers to lovers? Both will rely on different aspects to draw each other closer and in different orders.
Of course, sometimes plotting things out isn’t for the best. Your characters might grow together pretty naturally. Play it by ear if you want.