From the main Boulder Mail Trail, follow Mamie Creek about 0.8 miles until you get to the Mamie Creek Natural Bridge. Near the start, stay high about the south side of the creek, which cliffs out for a short section. After that, just stay in the creek bed until you hit the bridge - which you can't miss since the creek runs through it!
Flora & Fauna
If you keep an eye out, you'll be able to see lizards and other desert inhabitants.