Hi Dave,
Leather Cleaning is very easy to do. No machines needed or no wide range of chemicals to choose from..........just a bit of elbow grease!
You just use a standard leather cleaner and leather protector/conditioner. Most of the time you are cleaning a man made finish and so the dirt is just sitting on top of this and cleans off (in most cases) relativley easy.
The advantage of training is it allows you to identify what problems may occur on leather, how to avoid them and/or how to fix them.
As a rule of thumb. If the leather is absorbant it is going to be more difficult to clean. If the leather etains your cleaner on the surface its going to be easier. Other problems can occur on older leathers where the finish is weak for example and you may get colour loss or the finish may peel.
Problems like this can be avoided by testing first, you can then walk away........or training would teach you how to fix these type of problems.
You can get some basic information reading the guides here -
http://www.furnitureclinic-trade.co.uk/Guides.htm