Hike in the standard path. From the meadow past the cabins you'll see the obvious Great Arch flake arching up across the left side of the face.
The quintessential view of the Great Arch.
Unfortunately or otherwise, Arch doesn't start from the ground. You'll need to get up to the tree ledge by one of a number of routes. The two most popular are U-Slots (5.7)
or Entrance Crack (5.4)
. The former is, despite the rating, much easier than the latter and conciderably less wiggy. See the route descriptions for why I think this.
Arch itself is a 3-pitch affair; a right-facing dihedral the whole way. The first pitch is the steepest and cleanest with a full 50 meters of 1-1.5" finger crack down at the dihedral joint the whole way. A pair of bolts mark the belays and a tree provides additional pro as well as a welcome rest stance.
P2: The second pitch continues in the vein of the first, but the crack is more wandering here and is occasionally wet and root-filled. The slope is shallower and the wall to the left shorter. Continue upward to a second pair of bolts at a small tree.
P3 is quite easy as the slope relaxes conciderably and the wall on the left comes down to hip-height. Soon you're striding up the sloping horizon to the amazement and awe of the hikers lunching at the summit.
Descent: It is possible though cumbersome and rude to rap down Arch using paired 50meter ropes. This is a busy route and you're likely going to interfere with parties below. Please descend via the west-side walk-up trail rather than rapping the face. Easier and safer, and probably faster as well.
The pitches are a full 50 meters long and the bolts are placed accordingly. 50 meter ropes are essential. Bring as many 1-2" peices of pro as you can find; the first pitch is well-protected with a half-dozen #1 Camalots if you have them. Some slings to wrap trees are also nice.