Step 1 - Find a background image to paint on. Just like the wave tutorial, there isn't a series of steps to reach this point ;)
Step 2 - Roughly sketch in the placement of your trees, the main trunks only. One thing that's nice about trees is that they aren't "perfect". You don't need a precise, steady hand to make lovely trees. As would be expected, your trees will start out thicker at the base and gracefully taper off (some species of trees not so gracefully). Give your trunks a little character, don't make them completely rigid and straight - let them have their lumps and dimples.
Speaking of trunks, let's talk a little bit about them. You don't have to make a lollipop tree that looks like a ball of leaves on a stick. Some trees branch out very close to the ground, some have gnarled roots that poke out of the earth, some have limbs that spread out almost parallel to the ground. There are lots of possibilities so play around with it.
Step 3 - Because I have a little bit of backlighting going on in this scene I've gone along the edges of the trees and put in highlights.
There are tons of ways that a tree can be textured. Some of them are almost porcelain smooth, others have tons of whelps and knots. Even bark that looks detailed can be fairly easy to paint, as long as you're patient. I made three variations here and did a terrible little diagram for how I painted each one. 1.) Ridges in the bark streak horizontally around the tree, but go back in and make vertical scuff marks as well for more realism. 2.) Basically you just need to paint a bunch of blobs and shade the space between them. Make them random and varied in size. 3.) This is almost like painting sloppy stripes. Use different levels of brightness for the different "stripes", making some extra bright so they pop out a little further. Go back and add in some horizontal cracks to break it up a bit.
Step 4 - Now that you've looked at some bark, decide what route you want to take and start shading. I went with #1 because it's pretty easy and it looks nice to boot :D
Step 5 - My trees were kinda floating in a sea of nothingness there for a second, I anchored them to the ground with a bit of shadowing. I also went in to further refine with texturing of the bark with more highlights.
Step 6 - This is the second most scary part of painting a tree for most people I think (the scariest is yet to come! MWAHAHA). Now that you've got a solid foundation for your tree you can go in and start to add lots of small branches and twigs. The type of tree that I'm painting is fairly simple so I'm not having to do a lot of work here, but if your were painting say... an oak tree? You'd have a lot more work cut out for you adding in branches. Don't hesitate to look at photo refs to see what looks natural.
Now we get to talk a little about branches. Don't fear them! Love them! They love you! Don't make a big wiggledly gobbledy-goop of a mess. Think of trees as having... hmm... knees. Yes, trees with knees! Whenever a branch bends in another direction there is often a little knob there, like a knee. Even though the curves of a branch can look very graceful, they are often a bit knobby up close. Gives your trees knees and they will be pleased.
Step 7 - You know the most scary part I talked about? Yeah, this is it. The leaves. All those tiny little leaves. Well, it's really not so bad. All you do for this step is go in and make a bunch of dots. Is it a lot of work? Sure, kinda - but it's really easy. In areas where the foliage is really dense just color in the whole space and add dots around the edges for shape.
Step 8 - Ok, for this you're going to do something a little bit different. Get your lasso tool and "draw" out the shape of a branch with leaves on a small section of your tree. A shape kind of like this:
Using a slightly darker or lighter color, fill in the lasso selection. Why do you do this? A couple of reasons - first of all it makes really sharp shapes, and second of all you can be super messy and cover a lot of area with this technique. I usually work on another layer for this stage so that I can copy and paste my leaf shapes around on the canvas. Saving time/effort = win.
Step 9 - This is basically the same technique used in step 8 except you're going in and adding in extra bright highlighted sections of leaves. In these same highlighted areas I went in with a textured brush and added in spots of color to blend in the leaves a little better.
Step 10 - Last Step! Continue adding fine details until you get the look that you want. I went in adding more bright sections of leaves and some individual leaves falling to the ground. All done. Hope it helped :)
Here is another sample image using the techniques mentioned above.
