I used some leftover 12 ga. speaker wire, which uses many thin strands of thin copper wire.  I twist the whole wire for a short length for the trunk.  I use either a wood block or left over ceiling tile to hold the tree while working (wood works better).  As I got to a certain point, I soldered the trunk to lock everything together.  I then took a certain amount of strands, and twisted them off into a pair of branches, and left the rest to continue the trunk.  While the other wires were still straight (and relatively out of the way), I finished the lower branches by twisting for a length, then splitting and twisting more, all until it was done.

I then continued the process up the tree, until everything was completed.  The branching method, shape, etc. is determined by the type of tree.  One can model a particular type, or a general type.  Since this first tree, I've varied the branching methods to vary the looks of the trees as I've gone along.

After the branches were bent to the desired shape, I added solder to the branches.  This not only stiffens the branches, but fills in between the separate wires, changing the look of twisted wire to a more natural "knackered" look.  More bark lines can be scribed into the solder afterwards, if desired, but that will only be seen on the bottom portion of the trunk.  For the very thin strands of this particular speaker wire, it was better to lightly apply ACC to stiffen the very ends of the branches.  Wire with thicker strands won't require this.

Then I brush-painted the trunk with roof brown.  I've since used "rubber", "grimy black", and "weathered black" for variety.  If you are modeling a certain tree or general area, look at those colors for reference.

I then used Woodland Scenic's "Clump Foliage" and full strength white glue to add foliage to the tree.  For N-scale, the "Bush Foliage" is too large for my tastes.  The clump foliage stays clumped together for the most part, so I can add most of it to the branches with white glue dabbed on the ends.  Then, I fill out the tree (but not too well, and not too much) by dipping clumps into full strength white glue, and setting them on the tree.  The foliage that isn't in clumps can be set aside to be used  on the layout, so I don't need to take the time or effort to individually apply each and every clump to the tree.  Lastly, I spritz the tree with hairspray, then lightly sprinkle on other colors of fine ground foam (light green, olive green, dark green), and cover it again with hairspray.  This is a simple and easy trick, which helps to deepen the colors on the tree, easing the "plasticky" look of the mono-color foliage.  In looking at Eastern forests in real life and pictures, they are pretty much the same color, with only minor variations that can be accomplished with the ground foam.