Just make sure that the end result is small enough to fit in your oven! My tree (which isn't as large or good as the one pictured above) was a bit too tall and the tips of the high branches got a tad singed but you don't notice it after painting.

The wire armature is easy once you know the general appearance of your tree--just keep twisting wires together (be sure to sand the wires before hand to make it easier for the clay to stick to the wires).

Here's a tutorial I looked at before and thought was pretty good.
http://www.vhampyreart.co...10/01/polymer-clay-tree/