Clean the old thermal paste both from the cooler and the CPU. It is best to use isopropyl alcohol and cotton swabs or paper towel, but be careful not to go too hard on the CPU.

Dont use too much thermal paste, just enough to cover some portion of the pad. If you have a spreader, use it. The thermal paste will apply evenly after you tighten the heatsink.

You can also open the fan assembly to clean it better.

Last, put the heatsink back carefully and tighten the screws in order to ensure even pressure. Dont forget to plug in the fan cable.