Since Blender 2.70 I've been using Blender Texture Painting. Before slots I did not enjoy so much the process. I didn't get the results I wanted every time. Once a few fixes were added, texture painting is my most fun task in storybuilding. The writing and diabolical plotting is fun too, but there is something fluid and colorful about texture painting that cheers me up.
Not that I didn't need to find out what worked and didn't work. I did. It took several tries to make the process fun and repeatable. Now I use it for nearly every model I want to use for my storybuild, whether or not I decide to use surface nodes or just use specular and normal maps to go with.
Because its fun, and there is no need to make it less fun or this tutorial less fun, here are the bullet points:
Disclaimer: There are most always several methods for using modes in Blender Render or Cycles. Within each process there are choices to create internal processes. The process that follows is about making texture painting easy and fun. Once the basics are learned, it is best to add complication when you are ready for more involved outcomes.
1. Build a lo poly model or sculpt that you convert to a lo poly model.
2. Unwrap the lo poly model in Blender or Cycles. For this phase of the process Blender Render is my preference, because I can choose to vertex paint my model visually and easily.
3. Mark your seams and adjust the uv layout with the Lkey (less scattered layout). Clear your original seams if needed and use Seams From Islands to mark them again. Use Project From View if possible, but I don't suggest using the Smart UV Project, because it will scatter your uv into small islands. Some folks do use Smart UV Project, particularly for houses, but I fuss later about this wrap when I start to paint it. In other words, do as you please. This is your painting project, but if you don't like what happens, take a few extra seconds to lay our uv for painting.
4. Once you are satisfied with your layout, you can optional vertex paint it. If you decide to vertex paint your model, the map you make will be your first SLOT, or in painting terms, your ground. See below for simple Blender Render vertex paint steps
5. If you decide not to vertex paint your model now, go directly to Texture Painting. It is an option option like sculpt mode, object mode or weight painting mode.
6. When you choose Texture Painting you see your options (TOOL TAB). If you did not choose to vertex paint (causing you to add a material to your uv), then you need to make one now. Note also if you didn't unwrap your uv as suggested, you can use the automatic unwrap method that is time saving for uncomplicated projects. I rarely use this option, because I like to design my own painting template. Especially when I plan to "paint by number."
Go to preferences and change the color picker type.
10. Either use your per-made pattern outline, your vertex painting or paint directly from color picker or texture brush onto your uv map and/or model. If I used a texture (common license) for a layer of paint, I tile it first. I then stencil the same texture over the tiling, so that I can get the pattern I expect.. I either add it directly to the tiling layer or I make a new layer before I begin stenciling. Stenciling is also the process I use for adding decals, text, buttons or other details. It is another way to paint by number.