Well, going by the sound of the name, this subject sounds like there's a fair bit involved. Wrong. Piston ejection systems are dead simple.

At it's most basic level, a PES is nothing more than solid, anchored recovery wadding and looks pretty much like the lid off a can of spray paint or fly spray. You anchor the piston to the rocket with something like kevlar or more often than not, tubular nylon - fix it to the motor assembly if you can - and you attach your shock cord onto the top surface of the piston.

When the ejection charge fires, instead of pressurizing the airframe and popping the nose off, you are now pushing the piston up the airframe, which pops the nose cone off, thus deploying the parachute. The piston needs to be snug, but not tight - you want to push the piston up, not eject your motor at the other end ;-) The piston strap should be quite long...as long as a shock cord is good (too short and you risk zippering the airframe). Since there is a physical barrier between your parachute and the hot ejection gasses, you will never need to use wadding again, which makes it a desirable option for large diameter model rockets. For the small outlay in materials and time, it's probably worth it, but you will need to clean the inside of the airframe now and then to prevent the piston from sticking.

You can buy pistons to suit most airframes for only a few dollars, but if there is not a piston available you can easily make one by buying a tube coupler and a bulkhead (almost any mid/high power rocket manufactured will have these - a great source is either Public Missiles or LOC Precision). The piston used in my scratch built rocket as shown in the photos is an 'off the shelf' part from Public Missiles.