The following information is from an article from Oregon State Extension. See the entire article at http://extension.oregonstate.edu/gardening/.
In espalier training, tree branches are typically trained along the wires of a trellis, explained Ross Penhallegon, horticulturist with the Oregon State University Extension Service. The trellis may be of several types, depending on how difficult an espalier project you want to attempt. A wooden fence, wood, iron or steel posts, cattle or deer fencing or chicken wire can also support espaliered fruit trees.
There are several espalier designs, many with fancy names, including the single vertical cordon, the single horizontal cordon, oblique palmette with fixed limbs, Baldassari Palmette, the Belgian fence, lepage espalier with three branches, the U double, and the verrier candelabra.
All fruit trees can be espaliered, but home gardeners usually prefer apple, pear and plum trees. The tree must be in its first year or two of growth in order to be espaliered. Older trees are more difficult to train, as bending mature branches can take one to three years.
Any gardener attempting the espalier technique should be aware of the time investment required for success, advised Penhallegon. An espalier project may take from five to 10 years from start to finish.
Espalier training systems depend on a trellis, which may be quite elaborate. Build the lower, inner parts of the trellis first. Train the tree to it while the limbs are still flexible. Do not tie the ends of shoots down too soon. Develop lower, outer limbs before inner, upper ones.
Balance limb vigor by raising weaker ones, lowering stronger ones, or by leaving weaker ones upright until they catch up to the stronger ones, which you have bent down.
Ideally, the gardener should build the first level of trellis used to support the espalier before the tree is planted. The lower horizontal and inner vertical components of the trellis may be all that are needed for the first year or two. Since the tree may require six to 10 years to reach mature size, higher and more distant features may be added later.
If the trellis will be attached to a building, consider its aesthetic effect. Even though the trellis may be removed once the tree's scaffold system has reached its final, rigid state, the trellis will be in place a long enough time to support fruit load.
If the trellis is not attached to a wall, you will need sturdy posts on the ends for support. Use 4x4-inch or heavier lumber, or three- to five-inch posts. Set about one-fourth of the length of the post underground. A 12-foot post would be sunk three to four feet in the ground to support a nine-foot high trellis. Twelve- or 14-gauge, galvanized wire is best for trellises.
For more information on espalier techniques, check your local library or bookstore. Most gardening books include espalier sections, and there are books devoted exclusively to espalier.