Here is the basic cleaning process for longboard bearings:

Remove the bearing.

Remove the rubber cover.

Flush bearings with isopropyl alcohol, mineral spirits, gasoline,  or kerosene. Let them soak for a few hours.

Let them dry for about half an hour.

Replace or replace the rubber boot.

Replace the bearing into the wheel.

Don't forget your wheels

Just like the tires of a car, the wheels of a longboard need to spin for optimal performance. Try to prioritize wheel slip every few weeks. This process ensures that they wear out in an even fashion and that no single wheel takes the brunt of it.

When your wheels start to show signs of age and wear, it's best to replace the kit completely.