Park at the Wonderland South lot. Hike up the trail (breaking left) to Uncle Willie's, then head left from there to gain the main wash. Head north in the main wash for nearly 1/2 mile until the Wonderland Valley starts to broaden. You'll see Foolproof Tower on the left. Work your way west and a bit north across toward the obvious South Astrodome Formation, passing the very large Don Juan Boulder, then up slabs and through rocks to reach the broad northeast face of South Astro Dome. It is also possible to approach from the Barker Dam area, but I would probably only do that if I was interested in routes on the southwest side of the formation. (see Wonderland Valley approach map for more details to the point where you branch off to head to the South Astro Dome).