Pick a time when he is relaxed, stress free, and in a good mood. Make sure you two are in a private place where you guys can have an uninterrupted conversation. You do not want someone to walk in or over hear this conversation.  Avoid professing your love after a physically or psychologically charged situation (like before or after being intimate) because he may say that he loves you too due to an adrenaline rush or from being in an emotional environment.  Also avoid telling him if either one of you are intoxicated or sleepy. He may or may not remember what you said. If you discuss future plans for your relationship or how you currently feel, this is the ideal time to tell him that you love him. Be as natural as possible and tell him that you love him. Look him in his eyes and say, "I love you." You do not have to make it dramatic or awkward, just speak from the heart.  You can choose the ideal situation when you tell him, but try not to think about it too much. If the two of you are alone and having a good time, let him know. Listen to your gut when deciding to say "I love you."   Avoid saying, "You are the love of my life." It causes a comparison between you and his past relationships. He may love you, but may not consider you the love if his life at this point. You may be less likely to get the response you desire if you say that phrase. When you tell him that you love him, let him know that he does not have to say it back to you if he does not feel the same way. You do not want him to feel any pressure when you tell him.   You may say, "I love you. I understand if you're not ready to say it or you're feelings aren't in the same place as mine. I just wanted you to know how I feel." Remember that love happens at different speeds for people. Even if he does not say it back to you, it does not mean that he does not want to be with you.  Being patient with him is the best way to allow his love to grow if it is not already there yet. If your partner does not say "I love you, too," you can use this as an...

TL;DR in Vietnamese:
Chọn thời điểm thích hợp. Nói ra những lời đó. Cho anh ấy quãng nghỉ.