In Easel, you can create a simple ring shape by selecting a circle and changing the Cut Path type. The ring thickness will be the same size as the bit you have selected.
If you want to have greater control over the ring thickness, you will need to create a custom shape using two circles.
The first circle will be larger. The smaller circle dimensions will depend on the ring thickness you want.
For example, you will need to set the smaller circle .5" smaller than the larger one. You will then change the Cut Depth of the smaller circle to 0" and use it as a "mask".
Select both circles and use the alignment tools to center them. You should be left with something like this: