I would spend as much time as possible with them. Basically, i would try to really get to know this person.
I hope it helps?

Aku isi retro dia pake hal-hal yang aku pengen tau, jadi SA dia lah pokoknya

aKUUU ISI SC ATAU RETRO NYA SKSKSKS terus aku kepoin akunnya

I will openly ask to them and just go straight to them with the intention of getting to know them better. But I won't tell them out loud, just only me and keep acting like a friend. To be honest I wouldn't have made a quick confession, I'm a little shy when I talk about my feelings with them. I guess I just need a time...

I will just share anything with them. Ask their favorites, ask things that they like, ask about topics that they like, and bring up open discussions. Do my job as best friend and nothing more. If they show signs that it is positive, I will let them know about my feelings.