Even if you just want to make something that is super easy in theory, a lot can go wrong. It's important to know the size and dimensions of whatever you're planning to make. If you're going to 3d print at home, then you'll be limited by the size your 3d printer can print at. Helpful tools would be Digital Calipers, Rulers and a fabric tape measure.
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.