The trailhead is either of the two large parking areas near the summit of Guanella Pass. The trail to use is the one for Mt. Bierstadt on the east side of the road.
From I-70, take the Georgetown Exit 228 and drive east on 15th St. to a traffic circle. Proceed 1/4 of the way through the circle and exit south onto Argentine St. Drive south on Argentine then turn left onto 11th St. Turn right onto Rose St. and head south into the main part of town. Continue all the way to 2nd St. where you'll turn left and will be on SH 381. This is the Guanella Pass Road. Follow it south for 10 miles to the summit of Guanella Pass where there are two large, paved parking areas. On weekends, these can fill up completely and you'll find dozens of vehicles parked along the roadway. From Georgetown the road is paved all the way.
From US 285, take the turnoff for Guanella Pass at the town of Grant, west of Bailey. On this side, the road number is CR 62. Drive north for 12 miles to the pass and park where you find room. The road on this side is largely paved, but in generally worse shape than the Georgetown side.
National Forest campgrounds on the Georgetown side include: Clear Lake and Silver Lake. On the Grant side of the pass, campgrounds include Geneva Park, Burning Bear and Whiteside. All are fee-based and have vault toilets. Geneva Park and Whiteside do not have water. If you cannot find a camping spot at any of these (quite possible on weekends), then there are numerous at-large spots to choose from and car-camp pullouts. Just be careful to avoid private property and follow forest service rules.