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!
If you keep an eye out, you'll be able to see lizards and other desert inhabitants.