This is a trail that should be on every rider's bucket list!
There are two ways to start it. The most intrepid among us will climb the stairs from the north side of the Windy Ridge Interpretive Site and then traverse the pumice slopes, affording you expansive views of Spirit Lake. You'll dip into the woods before you cross the road to get the "main" trailhead.
Most people will stay on FR 99 past Spirit Lake and take the road all the way to the main trailhead, a quarter of a mile past the well-marked viewpoint. From there, the trail traverses a steep side-slope before quickly dropping into the Smith Creek watershed.
The top section requires you to surf some really loose pumice, so stay off the front brake and keep your weight back. After the pumice, you'll dip back into the trees and twist around and through a roller coaster of tight drainages. After the "Knife Edge" (you'll know it when you see it), you'll get spit out into Smith Creek.
At this point, some amount of bushwhacking is probably going to happen as the braided creek bed of Smith Creek invariably shifts and morphs every year with the spring melt. Do your best to follow the blazes and flagging to make your way across the creek, up into the aspens, and onto the decommissioned fire road converted to singletrack. This section of trail will divert up into the woods occasionally for some additional fun sections like using old growth blowdown as bridges.
Keep an eye out for the FR 83 Extension
at the end of Smith Creek Trail. This climb is the quickest and most direct route out of the canyon (there should be flagging). And the formerly ride-able trail beyond that, up to the bridge, and on the other side is eroded to the point that it's not safe anymore.
If you miss that turn, do not
turn on to the Lava Canyon Trail which 1) isn't legal for bikes and 2) has some very dangerous exposures that would require you to climb a ladder that could be exceedingly dangerous if you're carrying your trail bike.