Um actually a high idle mod is tricking the PTO to thining it is being used and it automatically brings the idle to 1200 rpm's.
Depending on wat year truck your talking about, if it has up fitter switch u can just use one of those and find the PTO blund cut wire behind the fuse box and hook up the two wires together and there u go.
Now if you dont have the up fitters dont worry you can still do it. u just need to get a toggle switch and get power to it the switch and hook up the PTO wire to the switch also and then that should work.
its pretty simple, if u search on the 6.0L general thread for a high idle mod u will find a full scematic