Ok,
Grab a toothpick or cocktail stick, an old toothbrush, small phillips screwdriver, plenty of kitchen towels and your vacuum cleaner.
turn pc off obviously
unplug cpu fan and and remove it from heatsink ,
use the toothbrush to brush the blades and the toothpick to get any welded on gunk off, once clean put your fingers on the middle of the fan and gently but firmly push to make sure its sat correctly on the motor housing,
a tiny blob of grease on the bearing doesn't hurt if you can get to it, or a spray of WD40 (lubricating spray) let it dry while you get on with the main heatsink.
Best way imo of cleaning the heatsink is to remove it, wash and dry well then re-apply thermal paste before reassembling and re-seating it.
but assuming you are leaving the heatsink in place,
Disconnect your rear fan (if fitted), just the plug, no need to remove the fan unless your cleaning that too
set your vacuum up so the hose is about 6-10 inches from the pc,(just close enough to have a bit of suction inside the case so it pulls the crap out) and sucking through the rear fans mesh/hole
gently use the toothpick to loosen the fluff/gunk/what the hell is that! from the heatsink, being careful not to knock the heatsink about too much,
now and again turn the pc upside down and let stuff fall onto the kitchen towels
you can use a rolled/flattened piece of paper to get further into the Hs if the design is a bit much for the toothpick,
once your happy with the state of it, do a final de-gunk inside the case, replace your fan *Remember to plug it back in*
and repeat as necessary whenever you need/want to,
hope this helps man,
Moto