Start up the crack below the obvious huge flake. Crank on the lieback, and work as much friction with your feet as you can from the slick rock. Work around the corner, stand on the flake, and finish up.
Protection
2 10 foot runners should work. For trad lead, bring your wide gear.
We've been rating this 5.10 for a long, long time, and after untold laps on all of the routes on Practice Rock think "Lieback" is definitely is harder than the route you call "Thin Crack" which is listed as 9+. Also, what is listed here as "Left Crack" is more like 5.8 than 5.9. I've been known to sandbag a bit, but I think more accurate ratings for Practive Rock routes left to right are 5.8, 5.9. 5.11, 5.10.
This is a great introductory route to Boulder Canyon, as a top rope can be set up, and trad may also be used. Be careful not to get your rope stuck under the slab or it may tear/fray ... poor rope.