To age leather (there's a difference between aging and antiquing) so it looks real is not a simple one or two step process - the process for me usually takes longer than the building. FWFW-I've been doing it studying how to do it for over 40 years and it's only in the last few years that I've really gotten the real McCoy Look.
Here's an article (maybe the one Noah was talking about) by one of the best that will get you started:
http://www.wrtcleather.com/1-ckd/tutorials/aging-leather-zurl.jpg
The best way to learn how to do it right is to:
1) look at real old/used leather and imitate the wear and look.
2) Make the piece new and then age it - doing it in parts and then assembling looks just that way, a hodegpodge of parts rather than a piece that is aged the way it would be done naturally.
To view some of the best aged leather work around take a look at folks like: Ken Scott, Jack Hubbard, Tim Albert, Dan Winkler/Karen Shook, (all are on line and that's just a few) and the artisans posted here:
http://contemporarymakers.blogspot.com/ - go back through the archived stuff as well.
Sandpaper and or steel wool will not really damage the leather when used properly. Crackling or crazing of the leather is in fact damage, but if limited to the surface that the leather will still retain it's integrity - crackling crazing is more applicable to antiqeuing (unless you're blowing them up like Noah
. Aging leather and rawhide some of the same methods but rahide takes certain specila techniques.
as for the difference in aging and antiquing........
Here's an example of a couple of antiqued pieces that are still eminently usable:
This rig on the other hand has been aged