Overhauling a freehub body can be a time consuming job. Always replace the ball bearings with new ones, as it's not worth re-using the old ones. When you buy the new ball bearings, it's worth taking the old ones to your local bike shop to make sure you're replacing them with bearings of the same size. Be careful not to lose any of the ball-bearings when they fall out of the hub, and that you count how many there are in case you lose any before you buy the new ones.
This is a long guide, so you can choose to either watch the entire video above, or follow it in smaller chunks by clicking on the individual steps below.
Steps to Overhaul A Freehub Body:
A freewheel is a ratchet mechanism mounted on the rear hub which allows the chain to drive the wheel forwards via a cog but when the wheels move faster than the chain the cog is not driven round by the wheel. A freewheel allows a rider to rest their legs while the bike is moving forward.