Seoul National University's registration process is the most confusing and stressful experience you will experience. You will appreciate and be thankful for STAR after going through SNU's registration.
Before the official registration, you can look through the catalog of past courses that were offered in the previous semesters to get an idea of what classes you are interested of taking and if transferable courses were offered as well.
A week before the actual course registration, you can pre-register for classes. This is not the official registration for classes, but just to get an idea of what your schedule can look like if you get all you classes. And while you are pre-registering, there isn't a timetable on the side like in STAR, so keep a paper and pencil next to you. If you pre-register for a class, but it becomes a time conflict, keep it in your "interest classes" list just incase for actual registration day.
On actual registration day, you can register for all your classes from the pre-registration list. But you need to make sure that you are in Korean Standard Time, meaning GMT+9. I remember it as (when in HAWAII), a day ahead, but 5 hours behind. There are two registration days, one day for odds and one day for evens; odds go first. But do not worry if you are an even number and is worried that all the odds will take your class. It's divided evenly during the two days. There are 3 more registration days just incase.
It is very important that you make sure to know the time difference as you do not want to miss your chance of getting into your classes.