I never did ride that trail, I hear it's tough! The one time I felt ambitious and went to ride it I couldn't find it.
Parts of it are tricky, but not any worse than other trails in the park. There are some really fun and cool parts of that trail, highly recommend checking it out. Took me a few tries to find it as well. According to Gary there are DEEP mines back here, I have no clue where they are though. Would love to find them.
Go in past Project USE, make a left at the fork (making a right would be going up Old Coal). In 2-3 minutes you'll see a little wooden teeter-totter on the left of the trail, stop. There should also be a rock ledge up on the right. This is where the beginning
and end of the loop are. To the right, going up on top of the rock ledge, you'll see one entrance to the loop. There should be two medium-sized rocks here, one on either side of the entrance. This is my favorite direction to ride the loop, although the first climb is tough. Worth it though.
If you want to ride the other direction, turn around (to go back the way you came) you'll see another entrance off to the right. Do not go up the little rocky chute right here, I made that mistake the first time. The trail you want heads off the fireroad at maybe a 25 degree angle, over some nice tree roots, as opposed to the 90 degree angle for the rock chute.
An alternative is to go maybe 70% of the way up Old Coal and keep an eye out for an unmarked trail on the left, around a left-hand turn. This trail is super tight through rhododendron, but pretty fun and intersects with the Wildcat loop at the end of that first tough climb, if riding the loop counter-clockwise. You'll ride into a campsite for Project USE, when you get to the campsite make a right-turn and continue on Wildcat.