Step 1: Ideas.
Note: I didn't have anything down on paper, this idea was on the spot.
Step 2: Getting Started.
Step 3: Getting Started 2
Step 4: The Background
First I started with a finding a city background on Google images. Pick one that you like, and go with it.
For my background, 5 layers of the background were used. First find a large portion of the picture you like. This should be the structure of the background. Now for the black space, , find parts of the image that will go well with the base structure. Keep doing this until you get something you like.
Erase to the bottom and the top to make it have that 'fading to black' effect.
Step 5: The Focal Point
The first thing you want to do is find a picture or render that you like. For importing your render, go to file, and use 'place'. This allows you to re size your render, and keep the quality good. Press control + T to free transform, and hold 'shift' to re size the render to appropriate size for the poster.