You need a rope to start. Circle the finger with the help of a rope and not leave it too loose because the ring will not fit exactly if the size is wrongly measure. However, it shouldn’t be tight enough that will cause pain in the finger. Just keep it well settled on the surface of the finger and drag it along as well to make sure that you can easily remove the ring. Sometimes, the size of the back of the finger is different from the front so keeping a slight precautionary measure can help to find a perfect size of the ring.
Use a marker or any other highlighter to mark the rope end where it meets the start of the rope. Make sure that no mistakes are made in highlighting the rope because that can cause the whole procedure to result in a failure and the time will be wasted plus the effort useless.
Measure the size from the start of the rope until the marked point and note it down in a copy. A millimeter difference can cause a huge difference so make sure that no carelessness is crept into the process which will just flush down the whole process.
Search the rings with the same size and try them before making a final decision. You can also try the rings with a slight difference and compare the feeling.
Always chose a ring that closely matches the size of your finger (calculated in the previous steps). Never make a decision in haste because you don’t want to rue it later so make sure the ring perfectly fits in and slides across the finger without any problem.