The next step in finishing my R2-D2 Holoprojectors was installing the lenses and retaining rings. To do this, I needed a pair of retaining/compression ring pliers which I was luck to find in the toolbox at home. The pliers can be purchased at a hardware store, but are a bit expensive considering you may only use them a few times.
First, just place the little lens piece in first. Then, sit the HP on a table upside down. Then, put the ring on the pliers and slide it inside the Holo. It should clap into the small grove cut out on the inside just above the lens holding everything permanently in place.
The first HP went together with no issue. The second only took two tries. The third was a struggle. For whatever reason, I could not get the ring to go into the HP right. In the end, I scratched the lens twice and the ring is only partially in place. I can't get it out without breaking the whole HP, so it will have to stay that way. I've poked and prodded, so I don't think it will cause me any problems.