NinePatch is a class that allows you to create nine custom patches that will be sized the way you define them. It draws an extensible image so you can resize it without losing the edges.
The amount of designs that can be created using this technique is virtually endless, so much so that Android also uses this tool.
To adjust the images and create the guides, keeping in mind that the corners will stay fixed, you need the help of a designer or someone with knowledge in image editing tools as illustrator.
But there are also sites that help to perform this work in a simpler way as this site. In it, just upload the desired image and then adjust the guides (always keeping in mind that the corners are fixed). That done, just download the .zip file containing the guides!