Mine is the ISP Decimator G-String II, it is the same as the original G-String with the addition of a link capacity to another one the same, maybe a few tweaks to a diode or something.
Once I worked it out it's really good.
You need a power supply.
You place it after your wah or guitar and then again in the loop before modulation effects.
Mine goes into the wah then into the ISP's guitar in jack. The out jack is connected to my tuner. From the send on my loop it does into an MXR 10-band EQ then into the Dec In jack on the ISP. After the ISP I have Flanger, Chorus, Delay, Boost then back into the amp.
If you were using with out a loop you would use the same order, but obviously the pre-amp would not have its noise suppressed by the pedal.
With a Peavey the pre-amp is the noisiest part, hence why it is good to have it in the loop, but maybe most of your noise is coming from pedals, so having it out is good.
My phaser is before the ISP (between my overdrive and a compressor) and works fine.