Pre-cut wood donut. Drill a hole that has a diameter that is 1/4" larger than the rosette. Then glue this donut on the underside of the soundboard to create a small ledge on which you can glue your rosette.
The sizing corresponds to the rosette size. Ex. If you are installing a small rosette you should get a small donut. If you are installing a jelly-filled rosette you should get a jelly-filled donut, etc.
I have not installed the donut, this can only comment on how it comes in the package. What will be visible is nicely finished, the non visible edge is not evenly cut, this will not influence the functionality.
Matt is extremely helpful and quick in responding to questions, 10+ stars for customer service.