From the parking area, head up the combined Herman Gulch/Watrous Gulch trail and then turn right after about 1500 feet (or five minutes of walking) onto the marked/signed, Watrous Gulch trail. The trail gains steadily into Watrous Gulch which it intercepts after about 1.25 miles. Once in the gulch, you'll emerge out of the forest into some avalanche debris and open tundra slopes. The I-70 noise will soon abate. In 1988, the trail began to fade out here, but now, a trail continues all the way to the Woods Mountain/Mt. Parnassus saddle. At about 1.4 miles, the trail crosses the stream and just after the crossing, you'll come to a signed trail intersection for the Bard Creek Trail # 83. Coordinates are: N 39° 42' 44.5 W 105° 50' 15.3". Close by on the Bard trail is a campsite area where someone has built a large wood/log lean-to type structure.
If you want to put the mind in neutral, just keep walking up the Watrous trail and enjoy the largely tundra stroll. There will be one trail heading off to the right not long after the creek crossing that is presumably a shortcut for Parnassus, however, that trail which follows a south running ridge line, may lead into avalanche debris. Use at your own risk. About 20 minutes after the creek crossing, there will be another trail intersection at these coordinates: N 39° 43' 09.50" W 105° 50' 25.7". Elevation here is 11,485 ft. To continue to Parnassus it is best to take the right fork. It crosses a small rivulet then comes quickly to a campsite located virtually on the trail. This trail will parallel another trail that stays down closer to the creek. Continue on the Parnassus trail all the way to the Woods-Parnassus saddle. For extra credit, consider a quick jaunt up and down Woods, which is ranked in the Top 700 summits.
At the saddle, turn right and gain the final 1,100 vertical over increasing chiprock, etc. as the tundra fades.
For a return, simply retrace your ascent route. For the more ambitious peakbaggers, it's highly recommended that you continue class 2 hiking over to Bard Peak, about one mile east, adding just over another 600 feet of gain. From Bard, it's possible to descend directly south toward the Bakerville exit on I-70, picking up a climber's trail that leads in that direction. If you don't locate the trail, this could lead to some serious bushwhacking. This descent will cross the Bard Creek Trail #83 which could also be taken back to the west to rejoin the Watrous Trail at the creek crossing.
For yet even more brownie points, the truly dedicated peakbagger can add Englemann Peak to their list for the day. From the summit of Bard, you'll need to walk north, descending the north slope of Bard to no-count Robeson Peak and continue north over mostly tundra to Engelmann. Then you'll need to reverse your route to get back to your vehicle(s), heading back over Bard. This is all easy and fast, tundra hiking. Adding Engelmann will increase mileage by 4 and elevation gain by about 1,600 ft. making this a much more ambitious day.
Links to other information, routes & trip reports for this peak that may be helpful.