After removing the wheel from the bike, remove the brake disc if so equipped.
Remove the cassette joint by lining up the yellow dots on the cassette joint fixing ring (turn counter-clockwise) which will allow it to be lifted off.

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Image on the above left is normal position, image on right is removal position.

Then lift off the cassette joint (note that the red dots are aligned).

One the cassette joint is removed, lift off the plastic driver cap. Underneath that you will see a snap ring that can be pried off with a couple of small thin screwdrivers if you don’t have the correct snap ring pliers. Now you can remove the sprocket noting the direction of the dishing.

Next remove the metal dust cap and the plastic chain guard which just pulls off. Now diligently clean and degrease all the exposed areas to prevent dirt ingress into the hub’s internals.

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Once everything is clean, just loosen the drive side dust cap and seal. NOTE, this has a left-hand thread, so clockwise to loosen – do not remove yet. It should be hand tight as it just has plastic threads, some rubber gloves or a rag help to get a good purchase on it.

Once the dust cap is loosened, get a container ready to catch the oil from the hub after the next step which is to remove the non-drive side lock nut and cone (15mm and 17mm cone wrenches required). Position the wheel hub over the container and remove the drive side dust cap and seal assembly completely, then you can fully remove the gear cluster from the shell.

Once the gear cluster is out, the oil will drain into the container, this is when you can see the build-up of metal particles in the oil, especially when the hub is relatively new.
Flush out the hub shell, non-drive side bearing and gear cluster with a degreaser (I use a citrus based degreaser) to remove as much of the old oil as possible. Then drain and dry the all the parts and clean off any other of the removed components.

Once you’re satisfied that everything is clean it is best to pre-oil the gear cluster before reassembly. I use a cut off oil bottle or an old drink cup or anything that is a close fit to the gear cluster. Just pour the oil into the cluster and let it sit for a short while. This oil can later be saved and reused as it is only pre-lubing clean components. Shimano hub oil – SG-S700 – which is a mineral oil, is stupidly expensive so after some searching around to see what other riders are using, I decided on using Royal Purple Maxgear synthetic oil SAE 75W-90W.
Caveat… If you decide on any other oil than the Shimano recommended oil, I bear no responsibility for the outcome, I can only attest to the fact that I have used the Royal Purple now for over four years and over 25,000 km Summer and Winter riding with no adverse effects to the operation of the hub. I did try a heavier weight of the Royal Purple but found that it made the shifting sluggish in the colder winter months.

While the Gear cluster is soaking, pack the non-drive side bearing with fresh grease and grease the seals in the cone and drive side dust cap.
The hub should be ready for reassembly now and you can either add oil now or with a syringe through the oil port later. My preferred method is to pre-lube before installing the gear cluster. To pre-lube, lean the wheel at an angle so that the non-drive side bearing and edge of the hub shell are at the same angle. Pour as much oil into the shell as will allow without leaking out through either side. Lift the gear cluster out of the soaking cup and let it drain off for a short while then install it into the shell. Now screw on the the plastic dust cap and seal, remember left-hand thread, so anti-clockwise to tighten and only hand-tighten. Those plastic threads will strip easily if overtightened!
Now replace the the non-drive side cone and locknut to the correct bearing adjustment, I basically aim for no end play as judging by rotational play is difficult due to internal resistance of the hub gearing. Make sure that is not too tight as the bearings and cone will be damaged in short order.

If the shell was pre-soaked, add an extra 25 ml of oil through the oil port then reinstall the oil port screw making sure that the small o-ring is under the head of the screw.
Grease the metal dust cap that was removed and refit to the hub.
Install the plastic chain guard, sprocket, snap ring and the plastic driver cap. Finally, install the cassette joint and fixing ring lining up the red and yellow dots respectively.
My hub has had about five oil changes now and the last two resulted in very little to none of the metal contamination that I experienced when the hub was new.

And that is the whole procedure. As I mentioned at the beginning, this procedure is best performed by a bike mechanic or someone with mechanical aptitude, but it is a very efficient method to ensure that your hub internals remain as clean as possible and should extend the life of the hub for many years of riding.