If you lay the bike over make sure your fuel tank only has a little fuel.
It might also be a good time to change the oil anyway.
How clean does it look on the dip stick after running?
There is indeed a seal there and get a cover gasket as well.
Not sure if there is a breather hose on the crankcase but check if there is because if it is blocked/collapsed it might be the reason the seal is bypassing oil in the first place.
Firstly don't lose the return spring they can fly along way.
You will need a pin punch to release the shaft so you can pull it out too.
It is all pretty obvious once you get in.
I have never replaced one of the seals, but life tells me these little seals are often painful to remove from the housing especially if it has a metal or hard outer case. Note which way the seal goes before you pull it out. That is why God invented cameras with phones attached.
Make sure the shaft doesn't have any burrs on it before you put it back in.
Lay you cover bolts out so you know which hole to put them back in.
DON'T OVERTIGHTEN the clutch cover bolts, if you have a torque wrench then use it and if you think it starting to feel the torque is still to much then stop.
From memory something like 6.5 ft lbs but check you workshop manual.
You really just need the gasket surfaces to be clean and just snug the bolts down in a criss cross patern so it tightens evenly.
Look at the parts page it may help.
http://fiche.worldofpowersports.com/yel ... 0-%20RIGHT
Any hassles jump back on with a question.