DYNASTY WARRIORS 9 Empires

This will happen naturally as you complete the scenario of your choice, you will gain grander titles with the more land you possess. Once you have fully unified China you will automatically become emperor on completion of the scenario.

There is also another method which I found when playing the 'A Gathering of Heroes' scenario where if you control the region of Luoyang (the capital where the emperor resides in the northeast of the map) you get the option to declare yourself emperor. (Having just tried on a new save I haven't been able to rush this by just invading my way there, I am guessing you still need to control many lands or be a Prime Minister etc. in order to get this option).

In the Yellow Turban Rebellion scenario Zhang Jiao is considered to be an emperor from the start, but as jimmymen has commented, this will not pop you the achievement if you start as him.