I always draw it out on paper, starting with the width of the fabric. Then draw lines to represent strips, squares, etc. until I have as many pieces drawn as I will need. As you go, you will see how much length in inches you will need. Then I always add a little bit, say 1/4 - 1/3 yard to allow for shrinkage or straightening on grain, mistakes in cutting, etc.
It always works out and I never have a problem.
I also take my diagrams with me to the store as a reminder of what I am doing, or if I choose a fabric of a different width, I can make the appropriate adjustments.
When I choose my fabrics, I always mark my diagrams with what fabric I picked for what strips/squares, etc. so that when I get all that fabric home, I remember how to cut it and how to place it! Sometimes I get scatter-brained!