It's simple really. All you have to do is be active and roleplay responsibly in the guild. A mod will notice this and will place your username on a list of people who should be promoted. If the promotion is accept by yours truly then you will receive a PM or it shall be posted in a topic (that hasn't been made yet) stating that you have been promoted.

Also, receiving the approval of a royal or high-up can get you promoted. In some countries you can also receive the title of a noble (which will soon be a type of sub-class) after doing a favor for royalty. This will allow you to get all snugly with the royals, which is extremely beneficial for your character. (And there's a possibility that I will make you a mod. This is not definite, though.)

If you have any questions post them here because I know that this explanation sucked and a mod or I will answer your question as soon as possible.