1 ) commonalities
the first thing you have to do is figure out what things they have in common. it could be only a few important things, lots of minor things, or any combination there of. they just need to have things in common in order to build a bond. possibilities: a soft spot for the same person/thing/cause, family history, personal goals, likes/dislikes, friends/enemies, hopes/dreams, fears, possible fates, etc. once you know what the commonalities are, you'll have to establish them in the story and show them noticing those commonalities.
2 ) attractions
whether their attraction is emotional, physical, or purely interest based, there should be some sort of attraction to keep them noticing one another and caring about the things they share in common.
3 ) tension
it doesn't have to be sexual or romantic tension (though it certainly can be), but there should be some sort of attraction-based tension between them, where they are having brief moments of letting their guard down in front of one another long enough to notice any commonalities and attraction. this is where the "subtle signs of love" come into play, but these can be signs of interest/attraction as well as "love."
4 ) bonding
at some point, something happens that allows a little bit of bonding to take place. this can be any sort of "moment" shared between them that lasts more than a couple of minutes. it doesn't have to involve conversation, but that's a good route to take since it allows one or both characters to share things that will allow a bond to take route. after that, little moments can continue to happen here and there, allowing the characters to bond further.
5 ) breakthroughs
finally, there should be some sort of breakthrough point where they both see each other at a true moment of vulnerability and begin to genuinely see the good in one another despite being enemies. this empathy allows the "flood gates" to open, so-to-speak, allowing them to embrace the feelings they have for one another. from there they only have to address what this new relationship means for their personal causes.