I write a lot of romance in the context of fanfic (hopefully the mere mention of "fanfic" doesn't put you off).
First off, presumably it's in there because you need it for plot development and/or character development (since the romance isn't an end in itself for you). So decide what it is you want it to do for you within the novel.
Then (counterintuitive) set that on one side. (I say this because for me at any rate starting with a checklist of "I want this scene to establish X, Y, Z about my characters" is the death of convincing characterisation... but you need it at the back of your mind for later.)
Now ask yourself, in world, what your characters would do? Is this a romantic attraction coming out the blue that's going to blind-side them? Is it unrequited or reciprocated? Is it the sort of thing that would happen in a rush, or slowly, with one or both of them being hesitant?
Write according to what they're like - then try to fit in the work you want it to do within the plot/story development almost as an aside.
And remember - less is more. The odd hint about what they find mentally or physically attractive about the other, dropped in at the appropriate moment, with most of it left to the reader's imagination works far better than lengthy descriptions of the fluttering of their hearts (if it's the romance you're focusing on) or each ripple of their abdominals (if it's the build-up of sexual tension you're going for).
And if it's overwhelming sexual attraction (I dunno, a torrid affair that's going to drive a pivotal plot twist in a thriller, for e.g.), don't feel you have to write sexually explicit stuff to indicate this (you can if you want, but you don't need to). When it comes to writing sex, even explicit sex, you actually want to get the reader 90% of the way there in terms of understanding how much the characters desire each other before the first item of clothing comes off!