You should also have a basic understanding of 3d modeling but I will be walking you through my process The concepts and techniques are not difficult to understand. You can also use any modeling software such as 3ds Max, Maya, or free alternatives such as Blender, Sketchup, Autodesk 123d, and any other programs of your choosing. I will personally be using 3ds Max.
You'll also want a few measuring tools such as digital calipers and a fabric tape measure. The calipers will help you accurately measure in mm for 3d printing and the fabric tape measure can measure the length of your wrist. You'll need to convert those measurements (inches/cm) to mm. You'll always be measuring when designing for 3d print!
Step 1: The Design
To start, you'll want to measure the width of your wrist. The cuff that I will be showing you how to make is a rounded cuff so the measurements won't have to be form fitting since a round shape is easy to get on the wrist. If you want to make a round open cuff bracelet that is more fitting to the wrist, then also measure the thickness of your wrist as well. Write down the measurements somewhere and keep it for reference later. You'll also want to have sketched out a basic design you would like or found some reference pictures of cuffs you like. Having some sort of blueprint on hand is important to keep you on track, even if you don't follow it entirely and just use it as a loose reference. It doesn't have to look that great either (see my example!).
Depending on where or how you will be 3d printing your round open cuff bracelet, minimum wall thickness will be an issue. Shapeways explains this thoroughly, I encourage you to read it. Also, to print in different materials like metal or ceramic, your model will need to meet the minimum thickness requirement. i.e. Shapeways can print in Stainless Steel as long as your model has a minimum wall thickness of 3mm. Also, the amount of detail in your final 3d printed piece is also limited by the materials minimum detail measurement so be sure to check with the 3d printing service that you choose for the exact specifications so that you can create your model with those things in mind. It will save you a headache later on and you won't get your model rejected from their service for not meeting the minimum printing requirements.