Calculating Power and Setup using resistors as heating elements
First concentrate on each of your 12 ohm resistors. Their max rated dissipation is 10 watts. The quickest way to find volts and current from power and resistance is max_voltage = sqrt(max_power * resistance) (or you could simply calculate current and hence power for each of several applied voltages). In this case, sqrt(10*12) is roughly 11v.

As you have a 12v supply, you need 2 in series, where the 24ohm total will give you 0.5A, and so 3 watts per resistor. With a 10w rated resistor, if you feel it's getting too hot at 3w, that may mean it's not heatsinked sufficiently, or your expectations need recalibrating. A 10w resistor would need to be big to be air-cooled, or to get very hot. If it looks like it can be bolted down, do bolt it to a heatsink. If it's ceramic with no flat surfaces, then it's supposed to run very hot in air.

Two parallel strings of two series resistors will give you a total of 12 watts. Is that enough for your application?

[hooligan alert] In my world (turn it up until it blows up, then back off a bit), 12v on a 12ohm resistor will give you 12 watts, which a 10w resistor will probably survive for a while, as long as you keep it cool. Four such resistors in parallel would give you 48 watts, enough? [/hooligan alert]