While riding on Memorial Day, I overcooked a corner and hit a tree. The impact blew out one of my fork seals, which I didn't notice right away but did notice later when I tried to brake and the fork oil all over my front brake made it not work so well. It was time to change the fork oil anyway, so throwing new seals into the mix wasn't a big deal.
This is not intended to be a comprehensive guide, just a walk through the process. If you're comfortable changing engine oil and filters and removing your front wheel, fork seals aren't a whole lot more difficult. Just make sure to consult your owner's manual for the proper procedure, as every fork is a little different, and it's entirely possible that I forgot something. This video is also helpful.
Put the bike on the stand and remove the front wheel. Remove the front brake caliper and the plastic fork leg guards.
Dial back the rebound to zero. Take note of the position of the fork legs in the triple clamps so you can put them back to the same height. Loosen the top caps on the fork legs. Then loosen the triple clamps and remove the fork legs.
Once the fork leg is removed, finish unscrewing the top cap. Compress the spring enough to expose the top of the damper assembly where it connects to the top cap and put an end wrench (in my case, 19mm) on the damper assembly. Put another wrench on the top cap and remove the top cap from the damper assembly.
Remove the spring, spring spacer, and damping rod from inside the fork leg and place them on a clean towel. dump the fork oil into a catch container where it can be recycled. Pry the dust seal off of the bottom of the slider. Remove the retaining clip from the oil seal and then extend the fork firmly to pop the oil seal out of the slider. Remove the seal and bushings from the stanchion.
Clean all the parts you are going to reuse (in my case, everything but the oil seals) and then reassemble starting with dust seal, then retaining clip, oil seal, spacer ring, bushing, and pilot bushing.
Compress the slider completely and add about 2/3 of the required fork oil. Pump the damping rod a few times to work the new oil into the damping unit. Then fill the fork to the required level, measuring from the top of the compressed slider to the top of the oil level.
Once the oil level is correct, replace the damping rod, spring spacer, and spring. Screw the top cap back on to the damping unit and tighten to the proper torque. Tighten the top caps into the sliders until they are hand tight.
Replace fork legs in triple clamps to the correct position and tighten triple clamp bolts just enough to hold them in place. Replace the front wheel. Once everything is in the right place, torque fork legs in triple clamps and then torque the top cap. Reset the rebound damping to the desired setting. Replace the brake caliper and the plastic.
Remove the bike from the stand and give everything a final check. If your brakes got oil on them, make sure to clean them thoroughly with brake cleaner. Give the bike a test ride to make sure everything is working properly before hitting the trail.