Right, you already know that the collector load has to be 1.2K, to give a gain of 12 - due to the 100 ohm emitter resistor giving negative feedback.
The base bias resistors don't really have anything to do with the gain, you simply choose them to give the greatest available headroom for the stage. So essentially you want the collector at half the HT voltage.
Knowing the collector voltage, and the value of the collector load, allows you to calculate the collector current. The emitter current is (near enough) identical to the collector current, so, knowing the value of the emitter resistor you can calculate the emitter voltage.
All you do now is calculate the base voltage, which is simply the emitter voltage plus 0.7V - calculate your base bias resistors to give that voltage on the base. Bear in mind that the base will draw current from this potential divider, so you can't make the values too large.
You can work out the base current, from the collector current and the Hfe of the transistor, you should pass something like 5 times this current down the potential divider network.