Make sure you have a ruler with cm and mm measurements as well as a pencil handy.
Begin by cutting a piece of paper into a 1 inch height by 6 inch wide rectangle. Place it on a flat surface with a lengthy width. Pay attention to the right and left ends. Reduce the height of the right end to 1/2 inch by cutting a quarter inch from the top and a quarter inch from the bottom - this is now your pointed end. Now proceed to the left edge and move in about 1 inch to the right. Poke a 1/2-inch incision in the center, equidistant from the top and bottom with the top and bottom edges intact.
You now have your homemade ring sizer. Wrap the ring sizer around the targeted finger's broadest part and push the pointed end through the slit. Note and note the location of the slit with the pencil; lay this along the side of the ruler and measure the length in mm. This is your ring size if you match the mm with the sizing conversation.