I decided that for this tutorial I will be adding a lot more detail shots throughout the process so that you can get a better idea of how loose my brushwork really is. A lot of times when people see my work they say "that is a photo" or "he painted on top of a photo," which I think is funny because my work is actually quite loose. I wanted to show that you can achieve a "photo real" illustration not by adding tons of detail, but by getting the shapes, colors, and values correct. If you can get these things correct you can paint very loose and still create a very lifelike and believable piece. There are tons of artists that do this, like Craig Mullins, Richard Schmid, and John Singer Sargent.
The first step is to always put in your main shapes! This will help you organize your composition and give you a starting point from which you can build your illustration. The brush I am using for the block-in is just a Photoshop default brush with the "other dynamics" set to pen pressure for opacity. I don't even worry about making my shapes refined, I just leave them very loose because I know I will be coming back and refining them later. I want to get this step done in a few seconds.
Using the same brush, I start laying down the base colors for the light and shadow side of the building. I make sure that I work at about 25-50% of the viewable file size and do not zoom in until much later (if at all). My working file size is around 2800x2000 pixels.