This is a trail that should be on every runner'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 heading down into the Smith Creek watershed.
At Smith Creek, there may be some amount of bushwhacking needed 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).
If you miss that turn, look for the Lava Canyon Trail, which has some dangerous exposures that require you to climb a ladder.