Basically you use some common sense, use your toys properly and make sure you communicate with your partner prior to starting. You want to stay away from exotic fluids (alcohol is bad unless you really know what you are doing, as are many types of astringents. Warm soapy water can be used safely, as can just regular water).
You also have to think about what type of toy you are going to use (sure-shot, water bottle, bulb/syringe). Usually it's not a problem getting the water inside... its the cleanup/expelling that presents your problems!
Will you be in a bathroom-style envrionment, or will your clients be getting the enema on the table and have to move to the toilet to expel? If you want to add a little more control and decrease the chance of leakage/dribbling, think about using a double-bardex enema nozzle. There are other types of inflatables as well.
For the most part a regular enema will just work on the items in the lower bowels. If you are trying to keep your play "clean"... have the client give themeselves a cleaning enema before they come to you so that you are getting to still play, but without all the accompanying messes. If you want a really deep-cleansing one, you are getting into the realms of a colonic, and I would highly advise you to stay away from those. Far too much risk for someone just getting into the sport.
Generally speaking you'll put 1-2quarts of warm water into them, and then they can hold it or immediately expel it. Some people will start cramping up very early on. Speed is not important here. A regular, slow flow will work just as well as a fast flow, and cause much less cramping and discomfort. You should make sure you can control the flow (either through pressure or a clamp), so you can stop and start as necessary. Don't worry if they can't take it all in your session, that's usually not that important. And you can always reload and try again. (heh!)
For general education here's the Wikipedia link:
http://en.wikipedia.org/wiki/Enema
Here's the link to Tristan Taormino's Puckerup page. It has lots of info on anal play:
http://www.puckerup.com/anal_advisor...nema_edge_play
You can even get books on enema play at Amazon:
http://www.amazon.com/Intimate-Invas.../dp/1890159514