Going to Celto next month! Anyone have any K-beauty recommendations I must try?

Hi girls! I'm going to Seoul for several weeks next month, and honestly, I'm planning on having some great skincare experiences. Thai people keep telling me that K-beauty is very different from back home in terms of crowds. However, I'm still not sure where to start.

I want to try those professional skincare services (like facial treatments) that are popular in Korea, but I don't want to take a gamble. Has anyone tried this before? Where are some safe and reliable places for foreigners to go? Any tips on choosing a trustworthy place would be great. I'd love to know what your experiences have been and what I should prepare. Thanks in advance!