There are a few different ways that you can install a retaining ring without using any type of tool. The first method is to simply push the ring onto the shaft until it is seated in the groove. You can then use your fingers to rotate the ring until it locks into place.
Another method is to use a piece of wire or a small nail to help align the ring while you are pushing it onto the shaft. Once the ring is in place, you can remove the wire or nail and rotate the ring until it locks into position.
If you have access to a Vise, you can also secure the shaft in the Vise and then use a pair of pliers to twist the retaining ring into place. Regardless of which method you choose, it is important that you make sure that the retaining ring is properly aligned before attempting to install it.