The aluminium ring is possibly what is known in the banjo world as the "flesh hoop" ... you'll need that to refit a new skin, but it's unusual, apparently, to find one on a plastic head. Maybe the plastic head isn't original and is a replacement for a previous skin?
Either way, it's all very straightforward if you follow the instructions to the letter ... clean everything, pre-soak the skin, fit the skin and just nip it down so's it's taught, let it dry
TOTALLY AND THOROUGHLY, at least 24 hours, the longer the better, then bring it up to tension slowly and evenly.
Do the job right and you'll have a skin that could last 100 years, get it wrong and you could be doing it again next week!
Deal with a reputable supplier. They should provide comprehensive written instructions and be able to answer any queries you have about your specific instrument, even if it means sending them some photographs to clarify a point. I've used
"Clifford Essex" in the past and would recommend them to anybody!
Good luck