There are a number of designs that ask you to divide the sheets into thirds and it is a process that scares some people! Perhaps the easiest solution is to divide into quarters, then remove (cut!) 1/4 from each side. This is wasteful and not very elegant. There is the "iterative method" which is accurate, but can confuse beginners. Other methods are suggested on that page. Here is a method of using a template with which to fold thirds onto a clean sheet of paper.
When you have a sheet divided into thirds, try to follow the "crease pattern" for making a pinwheel!
