Note: The mileage estimate for this trip begins at the upper trailhead at the end of the 4WD segment. If accessing this area with a passenger car and having to park at the switchback on CR502, add another 3 miles round-trip to the mileage estimate.
Since we have described the 4WD section of the road in the trailhead, this route description will pick up at the La Garita Stock Driveway Trail #787 on Trails Illustrated. Once you've crossed the creek, the trail heads south some and then curves into the unnamed drainage coming in from the east. The USGS map indicates that the trail quickly crosses the stream not long after turning up the drainage. What we found was that the trail stayed on the north side until the stream comes close to a steep embankment that makes further progress difficult. It crosses the stream just after you come out of an aspen grove. About 100 yards later, you will likely come to a stream junction not indicated on the USGS map. An unmarked stream flows down coming from the south and we found rock cairns indicating we should cross this stream as well and continue east up the trail.
The trail continues east and passes through a rough, very rocky stretch, passes back into forest and becomes fainter, but frequently marked with cairns. Most of the time, it stays well away from and above the stream, still on the south side. Eventually, the trail emerges at timberline around 10,750 ft. From here, you can pretty much chart your own route up the open, tundra/grass slopes. We went back across the stream here at a point where the trail bends SE and the stream heads NE and proceeded to a large, man-made pile of rocks. This point was about two hours hiking in from the CR 502 trailhead. We continued east for a little bit to a bench-like area then headed up a narrow ridge on the east side of a drainage. Eventually this pronounced ridge plays out to a broad, mostly tundra slope farther up. Continue on up to the ridge crest utilizing as much tundra as you can find. You should emerge at a saddle about a half mile south of the Phoenix summit. Proceed north along the ridge crest to the summit over mostly rubble. There's a very large, man-sized cairn/rock pile on the summit.
The summit view is interesting. Most of these mountains are not very "rugged," but dropping down on the north side of the ridge, there are still glacial cirques evident and a few lakes with some nice interesting, glacial carved features and cliffs. Farther down and to the south is the vast Wason Park. You may spot some elk in that area as we saw many droppings indicating their presence. To the north there are long valleys that stretch for miles.
For the return trip, simply retrace the ascent route or continue SE along the ridge crest from the summit for 2.25 miles to tag la Garita Peak. The route we have described here is mostly that offered by G&M and worked well for us. Trail conditions may vary however and make route finding more difficult so be careful about reading your map.
Links to other information, routes & trip reports for this peak that may be helpful.