So first of all you have to check the weather at your destination. This year, Seoul has kinda unusual weather going on. It is hot and humid. Even my korean friend told me to bring summer clothes for september. Actually he couldn’t give me any advice because the weather is changing within a few hours. I decided to take some tshirts, jeans, shorts, a rainjacket, 2 pairs of shoes (excluding the ones I’m going to wear) and of course enough underwear and socks. When you pack your suitcase, you should roll all the clothes. This will prevent all your wearables from crumpling.
Now the next point is optional: The washbag or toiletry bag.
You can save some space and weight if you buy anything you need at your destination. I mean things like toothbrushes, shampoos or deodorants. But nowadays you will find a lot of brands which offer „travel size“ products. They are cheap, light and small. I always recommend to buy two of every item, so you can put some in your carry-on luggage. It’s usefull for the daily life too 🙂