A good starting point, is to know what the image is going to be used for. This can do a lot when determining it's size/scale and how is should be printed if printing is necessary. As well as many other aspects of the image such as, "DPI" (Dots per inch) and "Image Resolution".
Another point to consider is the style in which the image will be rendered. Will it be a "Photo Real" type of image, a "Free Style" drawing or painting. There are many styles you can use, but what is wanted by your friend will tell you how to render the image. If you are left to using your own imagination to come up with a style, I would suggest you render the image in a style you most familiar with.
You're also being asked to incorporate text into the image. This is often a very personal chose and should be agreed upon beforehand. Along with it's size, placement and shape if any.
Considering if some parts of the image can be rendered separately and introduced later. Can help when parts of an image need to be removed, replaced or altered. Using "Layering" in your image for this process can be most beneficial when changes need to be made.
This is just a simple out line of how I go about putting an image together, and is in no way comprehensive.
I hope it's of some use to you and good luck with this project. I would also enjoy seeing your results if and when your project has been completed, as I'm sure others will as well.
