It's somewhat more involved than that, but relatively painless.
I bought a 2.5 gallon air tank from onboardair.com, however you can get them from Quadratec too. If you want, you can grab a 4 galloon tank from a big rig truck salvage lot too.
All of these tanks have multiple, threaded openings. On my rig they are as follows:
1) Air intake line from compressor.
2) Air output to quick air connect for airing tires, or whatever.
3) Brass plug to block on port off.
4) Safety valve
5) Air release valve.
I bolted the tank through the bottom of my tub under my passenger seat, protected by the frame rail.
Now the compressor I used is a stock Sanden from a jeep cj that had air conditioning. On the intake port I have a small air filter from a lawn mower engine. The out put air hose goes to a manifold.
A manifold is a piece of metal with multiple threaded holes in it. You use this to screw in things like your Pressure Gauge, the hose going to the tank, and most importantly the Pressure Switch. This is what senses that the tank is at 120psi and turns off the compressor.
Email me if you have detailed questions. email@example.com