As far as I’m aware, there aren’t any actual guides but I can give a quick list of broad hints for each RO (I’m mid-procrastination on something else so this is perfect timing!):
Ygraine: Probably the easiest RO. Say nice things to her but don’t go overboard with praise, she’s very practical. Spar with her at Tel Rahal and then join her to bathe. Later, once you’re a knight and provided you don’t join the Caliphate, she’ll approach you again.
John: Make sure you meet him by visiting the tavern while in Aleppo. Then either visit him at his lodgings or wait for him to turn up at Tebnine. If you want to remain with him at the end of the game, you’ll have to be either with the Papacy or a hedge-knight. He’ll regretfully leave you if you join the Caliphate.
Sana: There are a few different ways but the easiest is to make sure you visit Viv in Chapter 4 (if you at least try to look for her while in Aleppo, you’ll leave enough clues for her to find you) then ask to join the Caliphate. Take every opportunity to talk to Sana and bear in mind that she quite admires a bloodthirsty nature…
Isabella: You need to be with the Papacy and to have a high standing for her to even consider you. Then try to spend as much time with her as you can during Chapter 9 and agree to carry her favor at the tournament.
Loukia/Karim: The gender of your steward will swap according to your preference so they are always an RO. Just don’t be with anyone else by the time Chapter 6 comes around and you’ll have the opportunity to romance them provided you haven’t put them off by governing in too brutal a fashion.
Tonzo: You can “romance” him in Aleppo and Tel Rahal if you like, but he’ll back away when you become a noble. If you turn hedge-knight, however, you can pick up where you left off. At the end of the game, you’ll settle with him if you don’t have any other partner and if you have a history of romance with him from earlier in the game.
Ibn al-Ma’mun (hardly anybody seems to have romanced him so far which is a shame since he’s one of my favourites and he also writes odes to the player which vary according to their stats!): Sign up with the Caliphate then make sure you challenge the Lady Fatima on the road to Jerusalem to meet him for the first time (although you can still meet him later anyway). If your standing in the Caliphate is high enough (he’s an important figure), you’ll be able to romance him during Chapter 9.
Ali/Marjanah: The gender-swappable bandit chief isn’t a full RO but can be romanced briefly even if you have another RO. Go to speak to them when trying to clear out the bandits from Tebnine and be sure to be polite and settle the meeting on good terms. Then if you later become a hedge-knight and take up banditry, you’ll have the chance to team up with them for the summer.
Hope that helps!