Think ahead about how you want your case to look. It's important to get an idea of what you want your case to look like before you start gluing. Once the paper has been laid into the glue, you're going to have a gooey mess if you try to move things around.
When you apply your image to the glue, always press into the middle of the paper first and spread outward. This is a very important part of the process that will help to ensure that there are no wrinkles or bubbles on your finished case. You can use the edge of a piece of cardboard to help you smooth across.
DO NOT seal with another layer of glue until the glue under your image has had a chance to dry. Applying glue immediately on top can cause the paper to wrinkle.
Once you have an image down, you may want to apply more layers.