There's a seal around each end of the crankshaft, one behind the harmonic balancer and the other behind the flywheel or flex plate.
Pick the seal out with a screw driver (put it on the outside edge of the seal, tap the screwdriver down between the seal and the block and then lever it out.)
Then clean up the seal seat and crankshaft, maybe linish it with some medium-fine sandpaper and tap the new seal in.
Do that on both ends and you're right to go.
Oh and if you're ordering parts, give them your engine code (CA20E?) and ask for a front main seal and a rear main seal.