Guye Peak is the imposing cliff seen from I-90 while going east, just as you reach the pass. But that side is a rock climb, the scramble goes up the back side.
One good thing about this climb is that avalanches are historically rare on the route. But that doesn't mean you shouldn't take care!
See Commonwealth Basin for instructions on how to get into the basin area.
Upon entering the basin, stay to the west of the western branch of Commonwealth Creek. There is one point where you will have to leave the creek and climb about 40 feet above the creek, but don't go any farther from the creek than that.
Eventually you will find a creek which comes east from the saddle between Guy and Cave Ridge. You want to ascend the forested slope just north of this creek. Stay as close to the creek as practical, but don't climb the creek gully.
The gully ascends to a hanging bowl between Guye and Cave Ridge. Keep to the southern side of this bowl, and climb out of it on the southwest corner. You should gain the Guye/Snoqualmie ridge close to the north side of Guye.
Then go up to a large rock. It is often easier to ascend to the rock through the trees on your left. Move around the right side of the rock, underneath it. This is the most exposed part of the climb.
Once you pass the rock, just walk along the summit ridge to the summit. This is actually the North Guye Peak summit. The southernmost summit is slightly higher, but is much trickier to get to.
Follow the climbing route. It is usually possible to glissade directly from the big rock down to the area where you gained the Guye/Snoqualmie ridge.
There are usually lots of tracks leading into and out of Commonwealth Basin. Be sure to follow the correct trail back out.