I bought a bundle of floral material called Gypsophila from Hobby Lobby for $10. It was simply labeled "bleached gyp".
I start by trimming several small peices from the larger plant with a scissors. I am trying to make trees that are about 2" tall for my current layout project. This is a bit small for N scale, however, my layout is small. For larger layouts, a taller tree may be more appropriate.
In order to get a fuller or more "leafy" tree I found that gluing 2 or 3 peices together at the trunk works well. Once the glue is dry I spray it with a can of yellow paint.
Here I am applying acrylic white out of a cheap $1 bottle with a small brush to each trunk and branch. This doesn't have to take long, depending on how meticulous you are. I then used the end of a paper clip to apply acrylic charcoal to make the dark markings on the bark.
Here is the result: