When it says 'place the decimator' somewhere, in the case of the G-String that refers to the DEC IN/OUT jacks. The GUITAR IN/OUT part should go as far forward in the signal chain as possible. I like it just after the tuner, as I found that I was getting some funny noises while I tuned with it after, but I have it at the front, after the wah and the tuner, and the only reason the wah is right at the front is because it is easier to plug the guitar cable into the side of the wah pedal when everything is on the board if you are a right-handed guitarist like me. The DEC IN/OUT should go after all of the things that create distortion and noise but before the modulation and time based effects. If you have a noisy chorus pedal that kind of sucks, but otherwise you chop the chorus effect off.
With a high-gain amp like a Peavey 6065 series amp it should always be placed in the effects loop. If you have an EQ pedal in the effects loop I'd recommend putting that before the ISP too.
Easiest way for you to set it up (if you have a simple set up) is as follows:
GUITAR -> TUNER ->ISP GUITAR IN/OUT -> TUBESCREAMER -> PREAMP -> EQ -> ISP DEC IN/OUT -> DELAY -> POWERAMP
You know that the loop is between the preampp and the poweramp, right? With that in mind the diagram above should be pretty simple. When your pedal arrives plug it in that way. It should put a smile on your face. Just remember to use a power supply otherwise that smile will disappear in around 20 minutes when the tone starts to suck big time with the loop on and you can't work out why