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easiest/quickest
Get a factory service manual for your car and follow the directions to remove the transmission. Once the tranny is out you'll have to remove either the flywheel (manual) or flex plate (automatic). You might be able to get the old seal out of its carrier without removing the carrier from the back of the block. If so, drive a new seal into place. Use a seal driver so that you don't bend your new seal. If you have to remove the carrier to get the old seal out you'll need to remove a few of the oil pan bolts and may very well bugger up the oil pan gasket - be prepared for that. Change the oil seals where the axle halfshafts plug into the tranny while you have the axles out.
Personally, I'd pull the engine and tranny out of the chassis and do the job on a bench (or engine stand). You can change the front seal, oil pump seal, timing belt & tensioner, other belts AND do a very thorough cleaning of the engine bay and the engine itself.