You must first plan what exactly you are trying to make on the stencil. It can be either different shapes, or the same shape of different sizes. Just plan each and everything and let yourself know what should be the size of the stencil that you are trying to create.
After you are done with planning, it is time for you to draw each and everything that you have imagined. Try to make it in different sizes so that you can see which one is appropriate for you. You should know how much paper you will need and how you will be using it alongside the spray paint.
When you are done with drawing what you want to draw, try to print it all out. Printing will really help you solidify your ideas. You will also get a better picture and far more solid. You should also know that the shape that you have taken out from the printer, it will be according to the size that you have asked for in the computer. The print is not totally necessary but for perfection, you definitely need a computer.
Now once you are done with the printing, it is time for you to create the stencil itself. You should know which material will be used in the stencil. You can use paper too, but plastic will suit you better. Therefore, use a soft plastic which you can cut easily as well.
Now cut the plastic alongside the stencil. You will understand as you go along. The cutter that you will be using should cut all the outlines for the paper and for the designs that you have made.
After you are done with cutting the plastic and the paper, it is time for you to tape the two together. Tape them together and use a spray paint so that you can use it for whatever you have made it for.