Paint the background let it dry fully and then use a fan brush and start with your darkest shade of green first. I recommend to use 3 diferente values of green. For less color mixing allow each coat to dry fully between coats. Finish with the lightest value. This should be the areas where the light is hitting the tree.
The overall shape should be a long vertical triangle. Take your time be patient and with short vertical strokes create each branch going horizontally.
In the middle of the tree where the bark would be paint some random areas a little denser to give a look that those branches are shooting out straight towards the viewer. This will allow the tree to appear more 3D.
Remember to give space between your branches so that the birds have a place to sit. It's nice to be able to see the background coming through the open spaces of the branches.
Periodically clean and dry your brush so that the bristles will not be stuck together while you are painting. This will help create the illusion of many pine needles.
The biggest goal is to have overall cohesiveness being made up by a tons of randomness.