Replacing the rear main seal in a 360 without removing the crank?

Can you guys give me some tips on replacing the rear main seal on my 360 without removing the crankshaft? I've heard it can be done but I would like some tips from you guys that have done it before so I don't ruin the new seal. I haven't removed the old seal from the main cap yet but it doesn't appear to look like the same material as the new one. Also there is a small plastic tool in the new pkg. for installation of the new seal and I'm not sure how to use it. (Sorry for my ignorance.) Thanks in advance.