I had a leaking/weeping rearend seal and I watched the Honda mechanic change the seal. It looked pretty straight forward. Iasked him if there was anything to look out for if I ever did it myself, he said that just make sure the splines hook back you at the transmission side.
The honda shop manual advises you to remove the input shaft of the final drive (assuming you are rebuilding that part too, I suppose) and to use that short shaft as an installation handle when re-inserting the drive shaft into the front splines. Unless you are servicing the final drive too, just use a section of broom stick handle or toilet plunger handle to act as your extension for inserting the drive shaft, that way the fnal drive input seal and clip are not stresed. The fout nuts that hold the final drive to the swing arm are very tight and locktight glued - the upper left one is a bit of a booger to get to - but can be done.