You would first have to go to the parts store and but the main seal and see if it a one piece or two piece seal. If you are lucky enough to have the 2 piece then all you have to do is drop the oil pan and remove the rear main cap. You can use the new seal to remove the top of the old seal just by setting it up against the crank shaft and pushing it up over the crank. Them insert the new section in the mail cap ( making sure all the time that you have it inserted the correct direction and bolt the mail cap back making sure that the main bearing is clean and lubed. I like to coat them with a little STP oil treatment and a little motor oil mixed. Then reinstall the oil pan.
If you unlucky enough to have the one piece seal then you have to remove the transmission and follow the steps above.
