[NOS2Go4Me]

Here's what I do, and I've argued this method before ad nauseam on many sites(so use it if you wish) and I've NEVER had any issues in the 6+ years I've been doing it:

Find an old bank debit card or a cancelled credit card that's still in one piece. Failing those, find a slightly flexible, tiny spatula or other straight-edged utensil (knives don't work, even against the back of the blade).

Take your thermal grease (hopefully it's laden with pure, atomized silver) and put a dab on one side of the heatsink's contact patch. By side I mean in the centre of one of the 4 edges, about 1/4" from the edge or more depending on the size of the heatsink vs. the size of the CPU's heatspreader footprint.

Start spreading the grease across the heatsink. You'll immediately notice two things.

1 - there are fine grooves and nicks along the surface of the heatsink's contact area. This is normal. If they're excessive, you will not get the required surface area of the contact patch against the CPU's heatspreader. You'll need to consider sanding the contact area of the heatsink down slightly with a fine and ultra-fine set of sandpaper. Some folks even wet-sand with 800 grit and higher. I haven't tried it, but I'm not convinced it's that much better than dry sanding.

2 - the card / spatula leaves marks in the thermal grease you've spread across the heatsink. That's to be expected as the card can't be flawless. If the grooves are too deep, again... coverage may be an issue and thermal pockets may develop on the surface. It's unlikely that any real damage will result, but I tend to be a stickler for these things. Flip the card to the other long edge and try again if you wish.

As you go, work the thermal grease across the heatsink contact area such that when you're done (after having gone in all 4 directions) the grease is now a completely-covering sheen across the copper contact area. It should be just opaque enough that you can't make out the colour of the heatsink below it. If it's too thick, you'll see it in the raised profile if you were to look across the heatsink. Too thin and you'll be able to see the heatsink's colour coming through the grease.

If that's the case (too thin), dab a little more on the opposite side of where you initially started from and go again. Wipe one side to the other, and then crossways, before going back and forth again.

You'll waste some, but that's alright. When done right, it's a real champ to have on there.