From the AS website;
"Tint the bottom of the heatsink in the area where the CPU core will contact it. Put a finger into a plastic bag and thoroughly rub the compound into the base of the heatsink using both clockwise and counter-clockwise circular motion. This will ensure optimum filling of the microscopic valleys in the metal.
DO NOT use your bare finger to apply or smooth the compound (skin cells, and oils again)
Re-clean the heatsink surface with a LINT FREE cloth. Do not use any solvent or fluid. You may notice that the base of the heatsink is slightly discolored even after all the compound would seem to have been removed. That is the Arctic Silver thermal compound filling the microscopic valleys."