to shim, I used some these composite shims directly under the gutters themselves (see link below.) I made my rig with 2x6s. Technically, if you plumb everything to the top lip of the rig frame, then its only a matter of shimming the rig frame. But somehow it didn't work out that nicely for me... I built my rig right side down with this theory - maybe the ground wasn't straight, or maybe without gravity the u-shape gutter wasnt quite settled in the upside down method I was doing.
The other way of doing it is to shim the rig the best you can and then use plumbing straps to finesse the leveling of the gutter. Basically shim shim shim because I don't believe in attaching the gutter directly to the frame - lol. But if you don't have hang up then use barn screws to do it.
http://www.homedepot.com/p/Unbranded...2-TW/202807695
You might want to use a pressure reducer as well at the hose bib.