See main page for directions to the refugio. From the refugio the approach to the rim is easy. There are two choices.
1) Hike or hitch the 6 km road into the crater. Taking a vehicle up is 3 pesos per person.
2) Hike the wide trail from the toll both to a low spot on the crater rim and descend into the crater. At the rim descend to the left. The trail is obvious the whole way and is 2-3 km.
From the vicinity of Laguna de la Luna ascend the slopes toward the crater rim. At first there are multiple use trails, but they gradually combine into a large trail through the sand and scree that ascends to the left (east) to a low spot on the southern rim.
The summit, Pico de Fraile, is visible during this part of the ascent. Once ascencionistas reach the rim, views of the highpoint are blocked by lesser peaks. Follow the rim to the summit. Along the way will be easy trail and rocky sections requiring hands. The difficulties can be kept to a comfortable minimum but more adventurous groups can find more exciting terrain fairly easily. Some sections look more exciting than they really are once you hike up to them.
This route does not require much gear. Even when much of the mountain has some snow the ridge itself is often baked clear. Take you standard mountain dayhiking kit. Don't forget the sunglasses, lip balm, and sun screen.
I've indicated this route as "most of a day" but I am including in that estimate the hitch hike journey up and down the road.