This is a great trail when you just want to take it easy. It has it all; great views, easy climbing, and a sustained mellow downhill. By itself, the JEM is an out-and back trail, but can be combined with Goulds Rim, or the Hurricane Rim trail, to make loops over 20 miles.
The lower trailhead can be a little hard to find, but the MTB Project mobile app should lead you right to it. Traveling east on State Road 9 turn right onto Sheep Bridge Road (just before you enter Virgin, Utah). Cross over the Virgin River on the bridge and take the first right after the cattle guard. Follow the two track about a half mile to the end (this is the trailhead and parking).
There can be 2-way traffic, so keep your eyes open. Most of the time, there is a good line-of-sight making it easy to avoid problems.
The trail starts as you leave the parking lot heading west along the rim above the Virgin River. There are many great views of the river with Zion National Park in the background. The trail is obvious here, but after the trail bends south and leaves the canyon it joins a dirt road and it can be a little confusing (mile 1.3). Go right (west) on the road for about 100 yards until you cross a (usually) dry wash. Turn left after the wash and look for the brown trail sign to get back onto the singletrack.
At mile 1.6, you'll come to the Hurricane Rim trail junction - stay left on the JEM trail. As you continue, you'll cross a few double tracks, but the singletrack is obvious on the other side. At Mile 4.5 the singletrack crosses a well maintained gravel road. If you are doing the JEM as an out-and-back, cross the gravel road and continue on the trail on the other side. Follow the singletrack until you come to a super steep hill. This is the JEM drops. You can turn around here and start the descent back, or a little ways further up the hill and continue to the upper parking lot. If you continue on, you'll hit another short, steep uphill before the trail mellows out on the way to the upper parking.
Whenever you turn around, that's where the real fun begins. Since the trail is completely downhill the whole way back you can just take it easy and enjoy the desert scenery. It can be traveled in both directions, but most prefer to save the downhill for last. Some people do shuttle the trail so that they only have to run one direction.