This really depends the people involved. Is one of them shy? Bold? Perceptive? Cold? Emotional? I think you understand where I'm going with this. You have to take this into consideration before you ever write a romance. Don't make characters do things that are out of character unless you have a really good reason why.
Anyway, relationships can be broken down into stages. Sometimes. Like I said before, this depends on the characters involved. I'll try to generalize relationships but I will be giving you different examples of how these stages can be.
1. The first step is normally falling in love. This can be either the usual "love at first sight" or the slow get-to-know-you type of deal. This can either be mutual between each character (like the Romeo and Juliet type of romance where two fall in love within their first conversation. ._. ) This can be one sided where one falls in love immediately but the other takes their time as they come to know the other. Or they can have a slow romance where they take a while to love each other. (Some go the childhood friend route where the love interest already fell in love when they were young but this isn't really explained because it happened in the past. This allows you to mainly focus on the MC or the other character in the romance.)
2. The confession. This is the part where most mangas decide to stop because they don't want to write an actual romantic relationship. This a crucial part of a romance. How this confession occurs can also differ greatly. Sometimes is just one person saying they love the other and the other reciprocating. Other times it's one sided. This again depends on the characters you are writing.
3. Building. Now is where the actual relationship starts. Things normally start slow with the characters slowly getting more and more open with each other with their emotions. This can start with small actions such as hugging, hand holding (lewd >.>), or kissing (and by kissing I mean little pecks on the cheek or lips. Not full blown tongue or kissing *other* places.) Some couples don't normally do this in public until they become used to expression affection in front of others. Some just don't care. Again, depends on character (I'm sensing a pattern here.)
That's how romantic relationships normally unfold. Important thing to note is that for a relationship to work out, a few things are required. These things are:
- Love. It needs to be real genuine love, otherwise it will just fall apart.
- Honesty. Just be friggin honest with each other about things whether it's emotions, life, or just small problems. If people don't talk to each other, things start to break.
- Respect. Respect the other's space and feelings. Don't push them and let them go at their own pace.
(Note that these are for when you are IN a relationship. This is not for when you aren't actually a couple.)
Now here a few other things. How fast your relationship goes can vary tremendously. If the other is a complete dense moron, then things will be slow. If you both are quite honest, then you can get to the romance very quickly. Relationships don't just pop out of thin air. The characters must get to know each other well and be in love before a relationship can be truly called romantic. Like I said before, don't make characters do things out of the norm for them. If one is normally cold, don't make them suddenly warm at times without reason. If someone is a block head, then they shouldn't be suddenly perceptive about people and their feelings.
I hope this helps. Romance is a very nuanced thing and I generalized it maybe a bit much. People are very different and how they bond is unique. Don't force something that doesn't flow.