The first thing you want to do it draw out the basic design. I like to do basic shapes first to give me a good sense of placement without the clutter of details. Once the placement is down I then detail the image.
New as you can see here with the basic shapes you can now detail your obejcts much easier. This image depicts a person moving from a hotel to a jet plane.
As you can see here deatils of shading have been added along with the proper labeling.
This was colored in Photoshop the commands I used are:
Left side Tool Bar:
Marque Tool
Text Tool (After Clicking the "T" you can change its color at the top tool bar that appears)
Red Color Swatch (change you swatches to any color once you select them by double clicking)
Rectangle /Circle Tool
Paint Brush
Eraser Tool
You can use these tools as much or as less to achieve the desired result.