Make two sims socialize using the 'Friendly...' option. Once they become Good Friends, you should start using options from the 'Romantic...' category. I think that, personally, you shouldn't use...
Well, it really is easy.You have to have two Sims that interact A LOT.Just keep them interacting and interacting,until the relationship scale on the top left says the highest amount it can be;right...
Just keep doing some efforts. Go out with him/her, do things together, try to kiss or try other romantic things, some day you'll see the option to have a relationship. I've been rejected and now it...