When designing jewelry, loose diamonds are probably one of the the best ways to start off, since loose diamonds have a number of advantages over set diamonds.
They can be easily checked for flaws. A diamond laboratory can easily check the quality of the diamond as all sides and the bottom are readily available for inspection and analysis. When a diamond is set then the bottom is often hidden and the prongs or claws can hide serious flaws that affect the value of the diamond.
One can design the type and style of jewelry to suit you around the loose diamonds that you have. One does not have to be satisfied with a setting that is not really suitable.
First one has to get the loose diamonds. This will depend upon your budget and the type and size of diamonds you want. Then deciding on the precious metal to be used for the jewelry. Usually gold or platinum. Platinum is twice the price of gold but is harder than gold and more durable. Gold can come in varying degrees of hardness. 24 karat gold is pure gold and rather soft. Not really suitable for jewelry. 14 and 18 Karat is considered better for jewelry. The gold has been mixed or alloyed with other harder metals such as silver, zinc and sometimes nickel. This makes it more suitable for jewelry. Rings wear out more than other jewelry so this should be taken into consideration.
So having established the quality and amount of diamonds and the precious metal to be used then comes the design. This can be sketched out and then given to a jeweler to make up. Ensure you find a really good quality and honest jewelery, preferably one who is a member of the GIA. Ensure that your diamonds have GIA certificates (or equivalent) and that the diamonds match the certificates. Branded diamonds, although cost more, are better as they have the certificate number etched into the diamond.
Ensure it is very clear with the jeweler exactly what you want and how you want it. Check several times with them until you are completely satisfied.
Importantly with jewelry loose diamonds and making up the piece you desire, common sense and some due diligence are always a good idea.