First of all, we should define the purpose of thermal paste. It is used to increase the thermal conductivity between the chip (processor) and heatsink. While the thermal grease has low thermal conductivity, and should be applied in a thin layer for a more intimate contact between the two parts. Thus, it maximally fills the existing airspace. Choosing the thermal compound, try to avoid the imitations of branded brands that are well established. If you put a fake on the processor, may cause overheating of the chips, and breakage. Methods of checking thermal paste, and brands that are suitable for use in computer technology, you can easily find on the Internet.
To apply the thermal grease on the processor using a soft cloth clean the surface of the chip from the remnants of the old stuff. Do the same with the contacting bottom of the radiator cooler. Before applying the main layer of thermal paste can be rubbed into the surface of both the so-called zero layer material, that is, after pre-coat to remove it. Thus, the thermal grease will remain in the grooves and scratches on both surfaces and provide a better contact.
Squeeze a small amount of substance on the main chip. Some manufacturers suggest applying the thermal paste on the processor crosswise on the diagonal, and thereby achieve the best greasing of surfaces especially at the corners. Smear it on the CPU you need a small sturdy plate. I do not advise to do it with your fingers, but if you like more, use the rubber fingertips or gloves. The resulting layer of thermal paste on the processor should be fine.
Greater amount of substance apply onto the sole of the cooler and also spread it evenly across the surface. Then tightly secure the heatsink over the processor and secure it with locks.