RussellP90120,
I know how meaningful it can be for someone to make an engagement ring for their significant other, so I get that you want to make as much of it yourself as possible. I applaud your enthusiasm for embarking on this journey. One note of caution: metal art work is highly addictive. 
Once you try your hand at it, you will probably be hooked and want to do a ton more.
Julie’s suggestion about finding someone in your corner of the world where you could be as involved as possible is a good one. If there is a maker, goldsmith/jeweler, metal art instructor near you where you could work in their studio and get the guiding hand you need would be the best way to go.
The next best option is (if you haven’t already done this) go out to YouTube and watch several videos on the whole process of making a piece of jewelry through the casting, stone setting (if you want to include a stone or stones), and polishing/finishing process(es). There are a lot of them out there and some are better than others in this respect: they include the important, pertinent details to be successful. There are also several books on the subject. One in particular I have used is titled “Practical Casting” by Tim McCreight. I have worked as a gold, platinum and silversmith for 45 years and have taught metal art students for 30 and I will tell you the same thing I tell them - details matter. If you want to be successful in your quest - and who doesn’t, then you must pay attention to the details. Again, Julie is right when she says that casting is straightforward and a little bit technical, but once you understand the basic concept, and important details, you should be able to complete it satisfactorily.
When I introduce my students to making jewelry through the lost-wax casting process, because it is so novel to them, I break it up into 7 stages: wax carving, sprueing, investing, burn-out, casting, tooling/finishing, polishing/cleaning. In each stage there are important details, and each subsequent step builds upon the successful completion of the previous step or steps. For example, you could follow all the steps in the process, but if you incorrectly calculate how much metal you need, you could wind up with a partial casting and not understand why. Another example: you may go through all the stages correctly, but if you use the wrong temperatures while doing your burn-out, again, you could end up with a failed casting. In either of these examples you would have to start all over again at the beginning.
Before you embark on this task, you should have a clear idea of what you want the end product to look like. Here is a checklist of things I expect my students to know if I ask them:
- Do you have a sketch of your piece? Even a crude sketch/drawing is better than nothing because it is your road map. You don’t have to stick with it exactly as you progress, but it helps guide you.
- What kind of metal are you casting your jewelry in? Silver? Gold? Don’t even think about casting in platinum as a newbie. Waaay too many additional important details to attend to ranging from safety to procedure for someone new to the process.
- Will there be a gemstone or stones in the completed piece? If so, how will they be mounted in the jewelry? This is an art unto itself. So many different ways to set stones in metal.
- What additional work must be done after casting? If all you have to do is file/sand the rough casting, polish and clean it, then go for it.
5.If additional pieces must be added to complete the piece, how will they be attached? Cold connections (like rivets, or tabs, etc.)? Or will they be connected by soldering/fusing/welding.
(This will require using additional equipment of some type. Most commonly a jeweler’s torch.)
The moral of the story is do you research, find an experienced person to guide you (if you can), and carefully follow the steps in each stage of the process. Sorry for the length of this response, just trying to help you avoid all the pitfalls and be comprehensive.
Good luck and welcome to the journey of a lifetime.
Mike