Yonsei University offers three options for on-campus housing, from least to most expensive:
Double room at International House
Double room at SK Global House
Single room at SK Global House
Each type of housing has their own pros and cons. Important: If you want to ensure that you get your first choice, make sure you apply AS SOON as the application opens up (I suggest making an account before the application period). The application itself is easy and takes 30 seconds at most - all you have to do is chose the dorm type and click submit - but housing is extremely competitive at Yonsei. Definitely remember that the dates given are in Korean time.
The housing application opens up in around mid to late December, and the results are emailed about three weeks later. Unfortunately, I did not sign up as soon as the application opened and thus did not get my first choice, which was the SK Global House Single Room. However, in the end I was blessed and got off the waitlist for an SK Global House Double Room. For a while I was worried that I would not get off the waitlist because I don't dorm at UH and really wanted to experience the dorm life, but in early February a spot opened up.
Housing Waitlist
If you end up on the waitlist like I did (but hopefully not!), be on the lookout for an email saying that a spot opened up. It's not guaranteed that you will get off the waitlist, but if you do, you only have 1 day to decide and confirm that you want the spot. Once you confirm that you want the spot, your housing application will be updated and instructions on how to pay will be provided.
After I confirmed that I wanted to accept the offer, I was given 4 days to wire the payment. International wires usually take about 5 days or longer, so as long as you send your receipt as proof that you paid, your room will be secured. Just a heads up, there is a fee for wiring money - I use First Hawaiian Bank and the transfer fee was $50 + a $20 charge from CitiBank. First Hawaiian Bank uses CitiBank as their "middle man" to transfer the money.
While you are waiting to see if you will get off the waitlist for an on-campus dorm, make sure to do research about off-campus housing options just in case.
Off-Campus Housing
Since it wasn't guaranteed that I would get off the waitlist, I originally was going to stay at StayforU-Hongdae. Everyone who stays there is a student, and Yonsei offered a promo-code so it was a little cheaper than usual . From there, it takes about 15 minutes to get to campus by bus. You can apply for housing there without having to pay until about 3-4 weeks later, which I liked because I still wanted to see if I would get of the on-campus waitlist. Do note that if you are thinking about staying at StayforU-Hongdae as well, their refund policy is a little unclear. Their website states that you can cancel your stay without a cancellation fee as long as you do it 15 days or more before your check-in date. However, when I emailed them about cancelling since I got off the waitlist, their response was that they do not allow cancellations. Luckily my wire didn't actually go through since the banker typed in their account number wrong, but just keep that in mind. If you don't want to risk getting your money back, you should probably find another accommodation that definitely gives refunds.
Tip: If you are having trouble with finding housing, you should join the Yonsei Facebook group. A lot of students are willing to help if you have any questions, and some people even post offers if they need another roommate/housemate to split the cost. If you don't get on-campus housing, it's really up to you where you want to stay. The earlier you start researching for options, the more open spaces there will be (: