Fist up I sketched it out on paper to work out if it would work and what parts of the design needed to be altered. I had pictured this thing in my head pretty well, but this is a good step to see what issues you may have. Basically all that was needed here was to fatten up the thinner parts of the design and to add a keyring loop. As this was my first try at 3D printing I tended to er on the side of caution and thickened it all up a fair bit.
With my design mapped out, I headed to my 3D software to realise it in virtual space. I used Modo. While Modo is primarily a surface modeller used for illustration, (as opposed to a solid modeller used for engineering), Modo can export the .OBJ filetype which shapeways accepts. There are many other options like Sketchup, 123D and Blender. Use whatever you have and are comfortable using.
I started with basic shapes and built up the design from there. I didn't worry too much about the dimension until the end, basically I just worked the design till it looked right. I also turned the ring through 90 degrees cause it seemed to look better.
With a design I was happy with I then exported the file as an .OBJ and uploaded it to my Shapeways account. This verifies the model and tells you of any problems. i notices right away that the circle looked very jagged. This is a problem with using a surface modeller, they apply smoothing to your geometry to make images look better, but the 3D printer process will not do any smoothing, the geometry you upload is what you will get. I went back to the 3d software and added more points to my model and re-uploaded it. The model then looked smooth in the shapeways preview, however it told me the size was 1.5 inches high. I wanted it a bit larger so I went back again and rescaled it and uploaded it again. Now I had a 2 inch high model all good to go.
I selected the stainless steel material and completed the order process.
10 days later this arrives. I am very impressed with how it turned out. My partner was also very impressed with her personalised present.