From the Front Range/Denver metro area, head out of town on US285. Drive through Bailey, then over Kenosha Pass, dropping off the pass to the small town of Jefferson. At Jefferson, turn north onto CR54 (aka: the Michigan Creek Road) at the main intersection. The road heads first north, then turns west. In just about two miles from Jefferson, turn north again onto CR37 which is marked for Jefferson Lake. It's another 5.2 miles to the lake on a graded dirt road that most passenger cars can handle. There are two main parking areas at the lake. The USGS Jefferson quad shows a campground at the lake. This appears to be no longer the case, but there are vault toilets.
There are several fee campgrounds along CR37 heading up to Jefferson Lake. These are "Jefferson Creek," "Aspen," and "Lodgepole." There are also at-large camping spots. Since this is all located close to major metro areas, camp spots fill quickly on summer weekends. Plan accordingly.