Start of the route
This is the only route up to get up Darley Mountain that has a trail. According to a few other people, there are 2 other routes that exist that don't have a trail that some poeple use.
However, this route is conveniently located on the top of Engineer pass. The summit provides you will a jaw-dropping veiw of the entire San Juan range. There is an old mining road that acts as the trail, but stops randomly 100 feet shy of the summit. So getting to the summit requires some exposed scrambling on dangerously loose rock. You must know what you doing while climbing this.
, head NE towards Silverton. Once at Silverton go towards Eureka. Once your at Eureka, keep going until you see a 4WD high clearance vehicle sign. You will need at least some clearance to continue. Passenger cars are not appropriate, but a standard all-wheel-drive vehicle will make it if you are careful and go slow. Continue down the Eureka Gulch Road until you get to Animas Forks. Past Animas Forks, there is a junction for Cinnamon, Engineer, California, and Placer Passes. Head NE onto the junction for Engineer/Cinnamon Pass. A High-Clearance 4WD vehicle, preferably a Jeep, is required. Continue passed the Junction until you get to another junction. Take a left onto Engineer Pass. If you need in-depth details on going up to the pass, refer to a 4WD book. Before the top you will pass a Overlook that is about 200 feet higher than the actually pass. You can park at this overlook and hike about 0.1 miles to the pass where the trail starts if theres no where to park on top of the pass. You are very close to the mountain from here.
If you do not have a 4WD vehicle, but still want to hike this, I'd recommend taking Engineer Pass from Lake City.
Main route in blue
So starting from the top of Engineer Pass, there is a worn down path leading to a gate that is marked "No Motorized Vehicles". Take note of what it says and continue, by foot, up the mountain. The trail is very wide, but a great number of slides due to the extremaly loose rock, has caused the trail to be very narrow in places. Be careful while going across the slides. Once you get to your first switchback, make sure that there is no one bellow you just to be safe. Continue up to the next switchback. Then you will come to your last switchback. It ends abruptly and is marked by old mining artifacts. From here you are 100 feet shy of the summit. You will scramble up a slope made up of dangerously loose rock. Find the safest route while going up this, there are spots where you could slide off the slope if your not careful. But once you on top of the slope, the summit is to your lleft, about 25 feet above you. It is on top of a boulder but you could easily get on top of this boulder. Now you are looking at the entire San Juan range and parts of New Mexico (maybe).
On you decent, make sure you find the safest route down the slope in which you scramble.
Basic hiking essentials are required. A rock helmet may be a good idea.