Take Idaho state hwy 51 and turn just over the Nevada border heading west. Go to Garat Crossing. There are no signs so a good map (or topo map) will be required. The roads ultimately get rocky and high clearance may be required.
This is a remote section of canyon in Owyhee County. There are numerous canyons throughout this area and all are equally remote and spectacular. Autumn is the best time for both weather and road conditions.