You need to let him know that you want to have a serious conversation. This way, he takes what you’re saying to heart. If you don’t take the time to do this, he may not understand the level of importance of the talk. Let him down easy. Add a compliment or two into your talk but convey to him that it’s not enough.  “You’re a great friend, but we can’t be together.” “You’ll make someone very happy, but it’s not going to be me.” Even after giving him the reasons you’re not into him, he may not fully understand. Deliver your official “back-off” message after you’ve told him your reasons.  “We won’t have a romantic relationship.” “We can stay friends if you’re going to keep it at friendship.” “There isn’t the right chemistry between us.” If you don’t tell him flat out that your feelings aren’t going to change, he might hope for it. Don’t let him leave with any questions of you having romantic feelings for him in the future. Set some ground rules for your friendship (if there is still going to be one). Let him ask some questions if he wants to and answer the honestly. There’s no sense in protecting his feelings with lies: tell him the truth. This will help him move on more quickly. Rehearsing the conversation in your head helps, but can also hurt you by creating preconceptions in your head about how it will go. Rather than attacking him with your thoughts, sit across from him and hear what he has to say so that he will listen to what you have to say. To ensure that he’s gotten your point fully, ask him what he thinks about it all. Stand your ground and don’t leave the conversation without him understanding that you don’t like him. Don’t leave without clarity.

TL;DR in Indonesian:
Akui keseriusan percakapan tersebut. Bersikaplah ramah kepadanya. Berikan sinyal yang memintanya untuk “mundur”. Tegaskan kepadanya bahwa perasaanmu tidak akan berubah. Jujurlah kepadanya. Bersedialah mendengarkan. Bersedialah mengakhiri percakapan.