I think he is talking about a p68sc.
What Smiley said is correct, although short and to the point. Those are the ways to make it a CA gun.
The drop in front plug he refers to is a constant air adapter that goes instead of a 12g and the stock rod and cap. There are a few for sale here on this forum as we speak.
Dummy 12g, well it is what it sounds like, many for sale on ebay and other paintball web pages. will need to drill a hole in the bottom tube to attach the air line to the dummy 12g.
Tap the valve, Many examples of this in the Sheridan picture thread.
Which ever you chose you will need to mount a tank somewhere, the stock sheridan frame is not bottom asa friendly. If you want to keep the stock frame you will need a UMB which are often for sale on here or 360 days a year a Palmers.