Depending on the laws of a specific land which stamps to publish is decided upon in all sorts of ways. In some countries the government (usually through a ministry) decides what which stamps to issue, in other countries it is the postal authority or the postal services themselves. In Germany for instance the Ministry of Finance decides through two advisory bodies, while in Switzerland the process of done entirely within Swiss Post.
In (South) Korea stamps are issued by KoreaPost. However, before doing so KoreaPost must issue a document showing which stamps KoreaPost is planning to issue. Next people are allowed to object to any issue if they think there is something wrong with the subject (the designs aren’t known yet, so you can’t object to that!). You can object in a specific period in September (see document for dates etc.).