Different experts may have various opinions of the best strategy to design your own engagement ring, so that said, this is my advice.
First and foremost, the most important consideration is the taste and opinion of the intended end user of this project. If it is not you, then keep your goals focused on that person and don’t be distracted by other opinions (there will be many.)
Next of course is whether you have anything already in hand to work with. If you do, the design you make should feature that. Sometimes sentimental (family stones) or practical considerations should come first.
Budget will always come into play at some point. Having a budget goal at the beginning of your project will help very much in determining what choices will be relevant when working with diamond engagement rings especially. Some people are driven by choices of center stone (type, shape, size) and some by style considerations (vintage, classic, modern.) Other considerations such as micropavé and hand-engraving are better considered further along the process. Prioritize your decisions accordingly and the process will be greatly simplified.
Diamonds Vs. Other Stones
The great majority of engagement rings feature diamond center stones. There are round brilliant diamond engagement rings, cushion cut, emerald cut, marquise and princess cut diamond engagement rings, as well as other diamond cuts. This is often the best place to start in choosing your path.
Sapphire and ruby engagement rings are also popular and practical from a durability stand-point (MOHS hardness 9,) most often colored gemstones are not durable enough to take the daily wear and tear of decades of service on the hand.
There are an unlimited number of styles you can make your engagement ring in. There are ways to narrow the field a bit as well, which in most cases is a good thing. The first question I usually ask to get people to narrow their focus is what style most appeals to them? Choosing between modern style, classic style, or vintage engagement ring styles is a great first step. After that you can focus more on the details, which is usually the most productive way to solve any design challenge.
Finally consider working with a professional jewelry designer to make your dream ring a reality. Choose a designer that actually makes the rings rather that someone who orders them from catalogs. Anyone can call themselves a jewelry designer, but practice makes perfect, so work with someone who has been designing and making jewelry successfully for a while and they should be a great guide. If you do it right you will be creating a beautiful and lasting piece of wearable art.