How do I tell her about how I feel?

So there's a girl at work I really like who is currently in a relationship but they are ever so close to breaking up due to constant arguments etc, We are all good friends and we talk sometimes out of work and stuff... IF they do break up how do I go about telling her how I actually feel & How long should I wait... Obviously It'll not be instantly as she even said herself that she would be broken and at the same time if they were to break up, She wants to move on...

The girl and I are good friends compared to me and the guy.. Still friends, Just closer with the girl to the point of she trusts me enough with her "deepest" secrets...

So in regards to the actual question,
How long & How do I tell her how I actually feel?
How do I tell her about how I feel?
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