Differential gasket leak

So I guess, I've been popping it too much or the guy sold sold it to me this way but eiher way, I came upounhis leaky differential today. I have a membership at a local DIY garage. So my friend said he'd help me if I took it in so we could raise it up and replace the gasket. The guys ant Oil Express said it should be pretty easy. If anyone could tell me how difficult from 1-10 and some proper steps or a website I could buy the new gasket off of. This would all be much appreciated.