Sealed bearings are bearings that don't require lubrication due to the design of their setting. When they are installed correctly, sealed bearings keep out all but a minuscule amount of air. With standard bearings, the increased airflow allows a build-up of dirt and grime. The lubrication within sealed bearings is fed by the tiny amount of air let in and is maintained as it is used. Sealed bearings are commonly found in bicycle wheels; you can easily fit sealed bearings into your bike without anything but a sealed bearing set.
Set the wheel on a flat surface with the non-disc side facing upward. Use a hard surface like a wooden table.
Place the non-disc side bearing in the hub of the wheel. The hollow channel in the bearing should be facing up.
Put the socket face down on top of the bearing and push the bearing down until it is set in the wheel. Make sure that the rim of the socket is only touching the outer rim of the bearing when you push them together.
Place one of the spacers on top of the bearing in the hub with the flange flush against the bearing. Lay one of the felt seals over the spacer and push it down.
Secure the non-disc side bearing by placing the lock ring over the seal and tightening the lock ring down with a lock-ring spanner.
Flip the wheel over and insert the spacer tube into the hub. Set the remaining bearing down on top of the spacer tube.
Push the bearing down with the socket like you did with the first. Insert the spacer, seal and lock ring to complete the fitting of the sealed bearings.
You can buy a kit with all of the tools need to extract and set sealed bearings.
Only use sealed bearing parts that are designed for your specific model or the parts may not fit properly.