Well, there's a million ways to do leaves. So there's not an answer the way you asked. It's rather like asking how to paint water (well, maybe not quite so open ended, but still a little broad).
But if you are interested in doing trees, a way to do them without being a slave to rendering, whether you use stencils or not, the trick is the look at the overall shape of the trees. Each species looks different. And most change at least contextually with the seasons. And trees have a certain character depending on the needs of the painting.
If you are talking pines, they have a certain look and I think guys like Bob Ross painted them a lot because they are fairly easy. So check those tutorials out.
But if you look at architectural renderings, those artists put in trees that are decorative most often. And they have created something of a short hand way to indicate what kind of tree it is, that's fairly effective in the context of that style and they use them like props, so they have different qualities to enhance their building in a decorative landscape way. Those surprisingly have a value in learning from.
Also there are those that are done by old masters with their world of dense thickets and clusters of diverse species. What they often did is design a shape or several overlapping shapes that the whole business would fill, and then pop on mid-values and then lights, in many cases working dark to light. Many times they would be free handed in a generic way like someone might pop in clouds to their skies, so it looks like trees generally.
Some trees are dense and some are airy. And many times they can be designed or modified to work in a composition.
So think in terms of composition, and use some formulas when they're in the distance. You're not doing a portrait necessarily. And you could even go so far as to stylize the heck out of them so it may even barely resemble a tree, but still they would make a great painting. And that's what you're doing is making paintings, first and foremost.
What you want to do is look at Google Images and see what kinds of trees strike your fancy, and look at how they were painted. It could be like taking a walk in "The Forest" the year round.
Have fun. Air it out. And start looking and using that as a springboard for your creativity. That's what I did when I was learning or when I saw something that really caught my eye, or when I did it badly and wanted to expand my visual vocabulary. Your journey will help you because it's yours. And I would bet there are tons of YouTube tutorials on how to do them. It's very common to paint one form or another because they're all around us.
Go man go!
BTW, I enjoyed your entries into the competition. You showed many of them here as well so that's where I first saw them.
Last edited by D Akey; 05-18-2014 at 08:14 AM.
"Not a bit is wasted and the best is yet to come. . ." -- remembered from a dream