Say you have a mountain that is 6000 feet high and you want to land your helicopter on it. Assuming you know what ISA conditions are (as a noob
), in ISA conditions, your machine would think it was landing at 6000 feet. But say the QNH was 995 instead of 1013. There is an 18 mb difference which translates to roughly 540 feet. Now your machine thinks it is at 6540 feet, and the engine and rotors (or wings) have to work just that bit harder (in these types of questions, draw a diagram and place the larger pressures at the bottom).
Now let's look at the temperature. The ISA temperature at that height should be around 3 degrees C - but say it's actually 15 degrees - it is warmer than it should be by 12 degrees and the air is thinner (less dense), so your machine has to work harder still because it thinks it is another 1400 or so feet higher - actually around 8000 feet.
To find out density altitude, take the ISA temperature difference, multiply it by 120 and add or subtract it to the PA.