There are two main ways to gain promotion. The first, by far the most common, is straightforward enough: make a few posts (a dozen or so) in a week or two, without infringing any of the Rules. After that, you should be considered for promotion automatically.

The second, sadly much less common, method is to get yourself noticed by one of the Staff, as they can ask for your promotion to be manually fast tracked. Examples of things which may get you noticed are: writing a mod tool/guide, joining a Hosted Mod team, writing a gameplay guide or undertaking game mechanics research. IOW, demonstrating by your posting style/content that you are potentially a fine, upstanding Member.