How To Express Feelings To Someone's Lover?

You've asked a question about to express feelings towards his/her lover. But you didn't mention it that the person who wants to express his/her feelings is the one who is beloved or he is a third person. But I would answer supposing that the person who wants to express the feelings to the lover is the beloved.
So there is a very interesting way to express your feelings towards someone who loves you. You must show respect towards him/her. You can tell that special someone that you respect his/her feelings. And you like his/her concern about yourself. You must show gratitude towards him/her. It is the best way to express your feelings to your lover. You must not forget the famous words that actions speak louder than words. So keep them in mind. And also try to express your feelings with not only words but also with your actions. In this way, you can express the feelings towards someone who loves you. And who is concerned about yourself. Remember; don't hide your feelings because you don't know what tomorrow will have in store. So don't be lazy in expressing your feelings. And speak truth in relationship. It is the best thing you can do. I hope it is enough.

Do you mean How to express feelings to someone else's lover? Plainly you do not do  this. If someone you like is in a relationship, you should not try to draw them away from the partner that they are with.
Eventually, if you did, there would be some resentment for it most likely from the person that you want. There is deceit involved in taking someone else's partner, and if you are willing to do that, then what else are you willing to do. Or are you going to do this again with another person and dump the person that you drew to you initially. These are questions that rise when in a situation like this.
No by just me, but by human nature. Don't do it. If you think that the current relationship is not going to work, then you shouldn't have any trouble in waiting and respecting someone else's relationship.