The ring model consists of three pieces: two inner pieces made in part A and an outer piece made in part B.
To start, open up a new Tinkercad 3D design and drag a cylinder object from the right. Note that the number of sides should be 64 so that the inner ring is smooth. The two inner pieces are built so that they fit into each other so one is slightly smaller than the other. But the ridges on both (see 4th image) should be the same size so that the align smooth with the outer ring. The dimensions of your ring should be custom to whatever fits your finger but you can follow the dimensions in the photos for a general guide.
In this step, create the ridges of both inner rings (copy and paste makes this easy). Next drag over two more cylinders with diameter slightly less than those of the ridge cylinders. Then align each pair of cylinders, one new cylinder per ridge. Use the align tool to line up the cylinders x and y coordinates. Finally, use the group tool on both pairs. Look at the last image for reference.