Slit Trench Latrine
As the description says, this latrine is simply a slit trench. Use whatever is available to dig the trench, a can, sticks, whatever will work.
The trench should be no more than six inches deep and narrow enough to straddle. Make sure you dig at least 100 yards away from any water source like a river or pond.
After you are finished, make sure to cover everything with loose dirt. This will keep away flies, odor, and disease.