Over the past couple of months I've been asked by a few people how I do my digital backgrounds. I always reply with the same answer, "oh it's super easy". So I've decided to put together a quick tutorial on how I do my backgrounds.
Materials: Nothing too fancy, Photoshop CS3, Wacom tablet Intuos 2 (old I know), and a scanner to scan my initial sketch.
I'm going to start off by sketching my background on paper, it always feels a lot more natural to me and I can work a lot faster that way. For those who feel just as comfortable drawing in a tablet or have a cintiq, go for it, good for you.
My focus is going to be on the actual technique I use to render the BGs, but I want to briefly go over the sketch that I'm going to paint. My objective is to draw attention to the main character (marked in red), so I'm going to keep that area clear. The eye is also drawn toward harsh contrast, to I also have a bold horizon line bisecting the main character (marked in green). I make sure that any rocks or trees in the back create a line that draws you to the main character (marked in purple). Lastly my two secondary characters are contained within the shape of the rock as to unify them with the background and not draw attention from the main focus (marked in yellow). I've also noticed from my studies, that the horizon line is always right about in the middle.

Here are two important brushes that I use for my textures. They are literally bathroom sponges that I dipped in paint and scanned. You can make any texture into a brush easily by going to Edit-Define Brush Preset. I can send you hi-res versions of these textures if you like.
I've adjusted my sketch to be 8.5X11 and 300 dpi. I've the set the layer with the sketch on it to "Multiply" to make it transparent and put it at the very top so that I can see anything that I draw below it.
Now I'm going to start coloring it from the very back, which would be the sky. Before I put down the color of the sky, I make a layer then color it a solid light cream or grayish color using the bucket tool. Then I've created a layer of pink for the actual BG color also using the bucket tool. NOW, I set my eraser tool to one of my large sponge texture brushes, and lightly (20% or 10% opacity) go over the entire pink layer. This is what I'm going to be doing with every single layer I paint hereafter, this will give it that nice texture as well as unify the colors by making them slightly transparent (therefore allowing the colors beneath to show through and blend together).

Now I've gone and colored in all of the major elements in the picture. I did this mostly by filling in lassoed areas, I used a solid brush on the smaller trees and rocks. I've also erased over each layer with a textured brush like I did with they sky. Note that each element is on it's own layer. I like going back and being able to adjust my colors and fix things as I go along, so I keep every little thing on it's own layer.
Now I'm going to go back and add texture and details to each of those major elements. I like to do this mostly by going over large areas with a big brush set at about 40% opacity. I make sure to set the shape dynamics on them so I can get good varying line widths. Then afterwards I go back and use my textured erasers on them as usual. I will usually have 2 or 3 layers of texture for each element. One for dark, one for light, and one for small details like linework or spots.
These stones for example are one of the places where I DO actually paint on the texture. Generally I only add texture by erasing, but I paint it on in areas where I need it to look very deliberate. Some observations that I've made when studying old HB BGs were that spots or bark tends to be grouped into threes.

Now that I've finished adding texture and details to the individual components, I'm going to make one last layer over all of them, and add the last final details, such as tiny flowers and grass. (note that I've changed the color of the sky, I'm constantly changing my colors until I find something I like)
Lastly, apply hastily drawn characters, voila! finished.
I'd like to add that pretty much everything I learned about doing these BGs I learned by directly copying old Hannah Barbera (mostly Art Lozzi) BGs, and reading stuff on the John K blog. If you have any additional questions please feel ultra free to ask me. My hope is that someone out there remotely benefits from this tutorial. Thanks for your support.