Access the Thru Trail using Alberton Road
from the small parking area off Dogwood Rd. If that lot is filled up, you can try parking just off of Hollofield Rd, or further down by the metal bridge and RR crossing off Old Frederick Rd.
Start the ride on Alberton Road
by heading past the gate and continuing along the old roadway (closed to traffic). Look for a rock climbing wall and beaver activity along the way. Just after passing under a railroad bridge, you'll see the remnants of Saint Stanislaus Kostka church on your right, and soon Daniels Dam on your left. Around this point, the doubletrack will become singletrack and lead you to an active railroad.
Follow the trail on the river-side of the railroad tracks to a creek, follow that creek upstream underneath the RR tracks, then cross the creek before starting the climb up a rocky singletrack, passing a few vistas along the way before the track heads away from the river. This climb is the first of many along the way. You'll notice white blazes that mark the Thru Trail along the way.
After the first climb, you'll eventually hit a gate and gravel road. You'll follow the gravel road only briefly before following the trail off to the right, which will take you down a washed out trail descent that ends in a creek crossing.
From here, you'll climb through an open power line cut, back into the woods to switch back once or twice. Keep climbing, then descend, then cross a creek, then climb, then descend, cross a creek, ... you get the idea :) Continue to follow the white blazes.
The Thru Trail ends at a river crossing that takes you into the McKeldin area of Patapsco. During warmer months, the river is typically shallow and you can either rock hop or ride through it. There are lots of trails along the way that are worth exploring. Check out the Woodstock 101
ride for a recommended route.