Other posters have covered the general procedure: Look for any cases where you have two or more resistors directly in parallel with each other, or directly in series, and replace each of those with the calculated equivalent single resistor. Those new single resistors will make simple parallel or series combos, so apply procedure again until done.
But here's the trick to doing it in your head. Of course the series combos are easy -- just addition. At first glance the parallel combos seem difficult since the calculation involves reciprocals or equivalent mental gymnastics. However, you can often apply this trick:
a) N parallel resistors each of value R have equivalent resistance R/N. So 3 resistors of 100 ohms in parallel equal 100/3 = 33.3 ohms.
b) For parallel resistors that don't happen to be of the same value, visualize each of the resistors as parallel combos of resistors of some common multiple!
In this example, the first parallel pair to attack is the 6 || 12 combo in the right most branch. OK, the 6 ohm resistor is equivalent to 12 || 12, so overall what we have is like 3 resistors of 12 ohms in parallel = 12/3 = 4.
So now we have the right branch as 4+2 = 6 ohms, parallel to the middle branch of 3 ohms. 3 is like 6 || 6, so the effect of the middle and right branches is 3 resistors of 6 ohms in parallel, = 6/3 = 2.
And so on.