You press RB+LB+A, which will pull up a dialog line on the bottom left of the screen, if you have the chat pad you can type the codes here or if not press X and it will bring up the Xbox's keyboard. Then use these codes without the quotes:
"Engine.GrassQ 0" This turns off the grass.
"Engine.Shadows 0" This will turn off the shadows (it has to reset the game engine which makes your game freeze for about 90 seconds).
"Engine.SetFogParams 10000 10000 0 0 0" This will turn off 90% of the on screen fog (not the fog on the map screen).
"Engine.WaterReflections 0" Turns off the water reflections.
"Particles.SetMaxParticlesOnScene 0" Will turn off effects from spells and teleporters making them almost invisible.
"Game.Ragdoll.MaxSimultaneousCount 0" Removes the ragdoll effects.
The first two really make a difference, and the rest are just minor improvements beyond that, capital letters don't matter but only put spaces between the numbers. And in case you don't like the camera resetting on it's default position all the time:
"Interface.Camera.FCameraBetaSpeed 0" The camera will follow around turns but stop resetting itself.
None of these will disable achievements.
