Drive up and down River Rd. between Sattler and New Braunfels. There are numerous place that will allow you to fish for a small entry fee.
Because much of the river is tree-lined, if you're planning to flyfish, you'd be much better off with waders. It's tough to cast effectively from the bank amid the trees.
Fish the "green" water...i.e. the deeper pools and runs. I've never had much luck fishing shallow riffles in the Guad.
As far as flies go, consider fishing a dropper rig. Tie something like a Copper John, Beadhead Wooly Bugger, or Pheasant Tail to the business end of your leader. Tie another leader of 12-14 inches (2 or 3 lb. test) to the bend of the hook then tie on a size 18-22 nymph or midge imitation.
Personally, like as mentioned above, I'd rather throw hardware such as Mepps or Roostertail spinners at them. If you're fishing the the Trophy Trout section, consider mashing down the barbs of your hooks so that you can quickly and harmlessly release your catch.
Personally, I wouldn't use Powerbait or natural bait in the Trophy Zone. Trout are notorious for swallowing natural and prepared baits which means the morality rate is extremely high upon release.