Start by sketching out a trunk. My tree will be leaning, so it is bent to the right.
Now, add 5-7 main limbs. These are branching straight off of the trunk.
Add a couple background limbs. These aren't visibly attached to the trunk, but their location and size suggest that they are.
Lengthen the tips of the main branches. They should pretty much just be lines.
Now, add smaller branches off of the tips/main branches you just finished.
Darken the trunk/main branches so that they will be visible after the addition of all the smaller branches.
If you have any very large bare spots, add more main branches with smaller branches. Don't forget to darken the lines on any new main branches!
Now, draw long lines that will determine the approximate size and shape of your tree.
Now, go branch crazy! Just add many many MANY smaller branches, to fill in the shape you made in the previously step.
Where is the sun shining? In my picture, the light is coming from the upper right hand corner (metaphysically). Shade accordingly. Remember, wherever your light is, the shade is on the opposite side.
Sign your masterpiece, and BAM. Done. Doesn't it make you want to cry? I know, I know. You want to know how I can make a thing of such breathtaking beauty. Lets just say it's a gift!