Disconnecting the battery is highly recommended.

Unscrew the 4 screws holding the CPU heatsink and disconnect the fan.

Clean the old thermal paste from both the cooler and the CPU.

Use isopropyl alcohol and cotton swabs or paper towel, but be careful not to press too hard on the CPU.

It's also recommended to use compressed air to remove any dust in the laptop and the fan.

Use only enough thermal paste to cover a portion of the pad. Use a spreader if you have one.

The thermal paste will apply evenly after you tighten the heatsink.

Place the heatsink back into the unit carefully and tighten the screws in an "X" pattern to ensure even pressure. Don't forget to plug in the fan cable.