Dutch said you could go up a tier simply by participating on the world stage. So by being active in international topics. But obviously, things like helping end wars, helping settle disputes, spreading your influence over a large area, ect will also help go up a tier. Sheer force will too. So if your nation can get what it wants through force, it'll go up a tier.

Basically, if your nation is viewed as being very important, you'll generally be higher on the scale.