When carving a ring, always start with the ring size. The size may affect the rest of the design and all other measurements are in some way relative to it. 

Although you can file out the inside of a section of ring wax, this tool will make the whole process much smoother and more accurate. That said, always check the size of the wax ring on a ring stick. The measurements on the side of the ring sizer are not always reliable. 

To use, insert the end of the sizer into your ring wax and rotate clockwise or anti-clockwise, depending on which is most comfortable for you. Don't force the sizer, it's much better to use it gently and remove wax gradually, this will stop the blade jamming into the wax.