It is fairly straightforward; first you have to remove the pinion nut located in the center of the yoke.
In my experience, it is usually easiest to have someone hold the yoke with a pipe wrench/adjustable wrench and remove the pinion nut with an impact wrench. You could try just setting the e-brake and removing the nut with an impact, but the yoke will probably turn back and forth; preventing you from removing the nut.
Once you get the nut off, you'll need to pull the yoke out of the housing. With a little persuasion you can probably get it out; if it is stick a gear puller might prove useful.
After the yoke is removed, simply pry the old oil seal out of the housing; being careful not to burr the housing or pinion shaft. Make note of how it is installed so you don't put the new one in backwards (the spring should face the back of the car).
A large socket or seal driver will be needed to fully seat the new oil seal. Lubricate the seal surface with a bit of oil or grease and slide the yoke back in place. I am not sure of the specified torque for the pinion nut, you should probably reference a manual to see how tight it really needs to be.
Top off the gear oil and you should be done.
*Also, might as well change the rear U-joint while you're in there