There are two crank oil seals. One on each side of the crank. One is called the "stator side" and the other is the "clutch side". The names are self explanatory....
If you engine is smoking heavily white smoke and smells like burning transmission fluid (whatever type you might be running) then your clutch side seal is the culprit.
If oil is seeping out from the stator cover and your quad is idling REALLY high, the stator side seal is the culprit. Most people will recommend you replace both together and this is sound advice as one failing COULD be an indication the other could fail.... It is not, however required that you change both at the same time, they're not co-located....
For the stator side seal you'll need:
Flywheel puller, impact screwdriver with large phillips head bit, 1/8" drill bit, two drywall screws, and a bunch of patience.
The first step is to clear everything out of the way, you'll need to pull the stator side cover off which requires removing the shifter in most cases. You'll need to use an impact gun or flywheel holder and pull bar to remove the flywheel nut and washer. You'll need to pull the flywheel using the flywheel puller, and then use the impact screwdriver to remove the two screws holding the stator plate in.
That should expose the seal and the stub end of the crankshaft. The tricky part is drilling a hole in the seal on both sides of the seal body WITHOUT hitting the crankshaft or seal mounting surface. Hitting either one will cause irrepairable damage requiring splitting cases. Run the drywall screw into the holes in the seal body and then use a claw hammer to pull the seal out of the case. The new seal is driven in with a large socket that will fit over the crankshaft and drive the seal into the case. Be VERY careful upon installation not to drive the stator side seal in too far, it will rub the bearing inner race and tear the seal to shreds.
Clutch side has more parts to remove to expose but the seal itself is easier to deal with. Remove the right side footpeg, heelguard, and brake cable. Remove the clutch cover after draining the transmission fluid. Remove the entire clutch assembly one layer at a time and then jam the balancer shaft gear and primary drive gear with a penny and use an impact gun to remove the primary drive gear nut. Slide both gears, key, and collar off the crankshaft stub end and then pry the seal out of the case carefully. With the collar removed, the seal should be able to be removed with a flatblade screwdriver, seal extractor, or prybar.... Use the same socket to drive the new seal into the case paying attention to depth. Reassemble the clutch side opposite following the service manual for all of the adjustments.