Early models came without quick-disconnect fittings (QDs) in the fuel lines. You need to clamp off the fuel lines and then remove the lines. If you have original crimp-type clamps on the lines, buy threaded fuel line clamps before you do this.
Remove fairing parts as needed to access the fuel line connections, which are usually on the right side of the bike. Remove the retaining bolt(s) and disconnect the electrical connector. Unplug the vent lines.
Note: if you don't have the charcoal canister on the bike, it doesn't matter which vent line connect where. If you do still have the canister, notice that one vent line will probably have a series of XXXXX marked on it. This is the line from the filler neck drain and it goes directly to the ground near the footpeg. The other line goes to the charcoal canister. Some people mark the hoses before taking them apart. Regardless, make sure that the filler neck drain does not get connected to the charcoal canister upon reassembly. You can blow into the end of the line and feel for the air coming out of the hole in the filler neck. This line frequently clogs so testing it this way is a good idea even if you are not concerned about the vent line connections.
Install a shut-off clamp on each hose and then cut the crimps off with diagonal cutters. Alternatively, you may be able to spread them apart with a screwdriver to loosen them. Carefully pull the rubber hose off the plastic tubes from the fuel distributor, using a rag underneath to catch any drips.
While the tank is off, check that the crimped connections at the tank flange are tight (the hose should not spin freely on the metal stub).
Reinstall, remembering that the upper hose from the tank goes to the upper plastic pipe. And remember to put the clamps onto the hose first! When you do that, make sure you orient them so that the screws will be in an accessible location and will not rub against an adjacent hose or the throttle pulley.
If you have QDs on your bike, you just need to press the metal latch and carefully pull the fittings apart. Wipe dirt from the male portion and lightly grease before reinstallation. Press on the latch as you push them back together, to reduce the chances of cutting the O-ring. You simply need to be careful during this procedure.
When you reinstall the tank, take care that you don't lose the metal and rubber bushings that fasten the tank at the rear. These must be in place before you tighten the mounting bolt.
It's not an easy fix. The pinion shaft has to be removed and disassembled, which involves the use of a special wrench to remove the pinion retaining collar and a way to remove the nut from the pinion shaft (need to immobilize the pinion somehow; either in place by locking the ring gear, or by clamping the pinion gear in a vise after removing it). Plus you have to heat the housing pretty well if you are going to remove the entire pinion assembly. I nearly always combine this with a total inspection of the final drive, making this a relatively specialized process.
You might want to make sure the fluid is really leaking from the pinion seal. If you put different color oil in the transmission and the final drive, you can easily determine the true source of oil that's leaking out of the rear swingarm boot.
Some final drives (mostly Paralever models) came from teh factory with too much bearing preload, which can cause premature bearing wear and subsequent destruction of the bearing and seal. Chances are you're reading this because you think this might be happening to you.
Grab the wheel at top and bottom and wiggle it. If you have detectable looseness, it should be repaired. If you turn the wheel and feel anything other than smooth motion, you might have a problem. If you drain the gear oil and find any solid metal debris stuck to the magnet, you have a problem.
If you have fluid leaking from the boot between the swingarm and the final drive, you have a minor issue that may not be worth dealing with. It's about two hours labor to only replace that seal. If you'd rather pay that than keep wiping the oil away, have it fixed.
If you have oil leaking from the seal where the wheel bolts on, you can try simply replacing the seal. It is easy to pry out and the new one will install surprisingly easily. If the final drive exhibits other problems, replacing the seal isn't a good idea.
Fully rebuilding a final drive is 3~4 hours usually. I'm fully convinced that a properly built final drive is very, very unlikely to fail again.