I've found that the key to making a great looking character costume is to have it scaled correctly. Without a Creeper action figure (does this even exist?) to measure, we downloaded one of the many papercraft templates that are available online. Papercraft templates are great in that they give you all of the dimensions you need on a single sheet of paper.
Once you've gathered the dimensions, you will need to then determine your scale factor. Since this costume is for my daughter, we took our key scaling dimension off of her. Our scaling dimension was measured from her shoulders to the ground. The thought is that the body portion of the costume is supported by her shoulders, and as a result the shoulders of the Creeper costume need to match this height.
The scale factor can then be applied to all of the dimensions from the papercraft template. You are now ready to start cutting cardboard.