Hi, i am a geek in the department of electronics so..
what equipment do you need, what is the cost?
and how do you generally install it on your machine?
(if there is a general way)
thx

Murphy's instructions here outline the steps, as do Marshall's here and mine here.

Both of those projects involve adding a PID to a machine that is thermostatically controlled, and requires disconnecting the thermostat.

If you find yourself PIDing a machine that is controlled by a pressurestat, I'm a strong advocate (especially if using a home-rolled PID versus an industrial controller) for adjusting the pressurestat to a higher-than-you-expect-to-see-but-less-than-blowing-up setting; then run it in series with your solid state relay. If the SSR fails closed or the PID crashes (not likely with industrial controllers) the pressurestat will take over and prevent a boiler overpressure release.

It's just another layer of safety in the design and it's already there, so why not use it?

Perfect!
thx