It's not that bad, really. It'll take you a few times to get the hang of the steps, and one is kinda tricky, but... My recomendation is to avoid flats in the first place and go with a serious puncture-proof tire like Schwalbe Marathon Plus or Specialized Hemi Aramadillo. Best offence...
One thing to really pay attention to is the tightening torque of the axle nuts- they need to be tight! I've seen a few bent drop outs resulting from under-spec tightening, so get a torque wrench and feel what 25-30 ft/lbs is like.
Here are the service docs form Shimano for the Nexus 8(pdf)
The one thing that seems to be missing from the Nexus 8 instructions is the install/removal of the shifter cable. I would assume it is the same as with the other Nexus hubs, so
here is the pdf of the Nexus 7
The Nexus really is a thing of beauty and you will soon really appreciate it (especially if you never have to take one apart ;-)
Happy Riding!