Note: The Forest Service plans to restrict access to Monte Cristo for three years, beginning in 2013. What effect this will have on accessing this mountain, I do not know. A new road will be built on the east side of the river towards Monte Cristo. When this road is open to the public, it will actually make this climb a bit easier.
Follow the Mountain Loop Highway east from Granite Falls 30.5 miles to Barlow Pass. Park here, and follow the gated gravel road towards the old ghost town of Monte Cristo. After about one mile, the road is washed out by the Sauk River. Cross the log "bridge" to the other side of the river. After crossing the river, travel downstream about 1000', where you will see a creek bed entering the river from the right. You can follow this creek bed uphill if it is dry (It will become a slot canyon after about 0.4 miles and force you into the woods to the left). If not, enter the woods on the left (north) side of the stream, and start uphill. Refer to the photo below: The gully/creek below Sloan Peak, is what you will initially follow. The climb will finish in the gully that extends down from the summit area (the leftmost of the major gullies in the photo). I will refer to this gully as the "summit gully" in the route description.
Continue up the hill, which will become steep in a hurry. Fortunately, the woods are quite open at this point. The duff on the ground is very slick - the one piece of gear I wished I had brought with me is my instep crampons (sounds weird, but it would have made life much easier). I was able to find game trails for much of the route up the ridge, which did help a bit. At one point, I ran into a large goat - he was probably wondering what I was doing on "his" trail. The small cliff bands on the ridge are easily negotiated (usually to the left). As you ascend to about 5000', you will come to a large continuous cliff band (it appeared to me to be at least class 4 to continue straight up). Make a short traverse to the left (north) to intersect with the summit gully (ignore the two or three smaller gullies you might cross, the "summit gully" is large and obvious). I entered the gully at 5200' (N48 01.379 W121 25.312). As luck would have it, this was the highest point I could have entered the gully without cliffing out.
There is a lot of loose rock in the gully - use caution so you don't kick rocks at your climbing buddies. As you ascend to about 5500', the gully will neck down, and you will have a view of the false summit (200' west of main summit).
There may be some temptation here to exit the gully to the left to finish the climb. Unfortunately, this will lead you to the false summit, and will make your day much more complicated. Continue up the gully to about 5700'. At this point, you will be able to exit the gully to the right, and continue towards the summit on steep heather.
The final scramble to the summit is fairly simple, with minimal exposure, and a couple of class 3 moves. The final climb to the summit begins at the small tree in the photo below.
Sheep Mountain appears as though it would be a fantastic viewpoint. Unfortunately, the Eastern Washington fires made my views very hazy.