Basic knowledge is fine for working on your parents system. However, if your going to be installing systems for $, you need to know what you are doing. The first several systems I installed were designed by someone else. After doing a few, you get an understanding of how to place heads and what a well performing systems "looks" like. Without the knowledge of flow, pressure loss, etc. your really asking for trouble. The link that was given to you is great. Read it, design a system for your first project (own yard, family member etc) then have someone else do the design. Century Rain Aid, (now, John Deere Landscapes) may still do free designs if you purchase your supplies there. The head placement may need tweaking, but the pressure, flow, etc will produce a working system. Compare the difference and ask questions.
My first install was at my house. 14 zones. This was seven years ago and the only fixes have been nozzles, 1 diaphragm on a valve and a couple of spray heads that I hit with the aerator. Since installing that system I now do things a little differently and have learned trick to save time and money. However, the system works great. General knowledge may not produce the same results.
For Example: I've been asked to help friends troubleshoot their systems. Seemingly small problems, (like not enough coverage in one area), are usually bad design issues and not easily fixed. Bad systems are most likely the result of someone trying to save $ or lack of knowledge. Since you already do lawn maintenance, etc. take your time and learn how to install and design the right way. Provide great customer service and then don't compete solely on price. OR, don't take anyone's advice, make mistakes along the way, and have a questionable or non existent reference list.