The rosette is made from veneer which I glue around a wooden cylinder that has the diameter of the inside of the rosette. The resulting wooden tube is cut to individual rosettes on the table saw. In order to have as little loss as possible, I use a very thin sawblade, the same I use for fret slots as well.

The channel for the the rosette I cut with the router. 

After the rosette is installed and the glue is dry it is sanded flush.

Then the top has to be sanded to the desired thickness which is somewhat below 3mm. Depending on how stiff the wood is, it can be a bit more or less.

Finally, the soundhole is cut and the top is ready for bracing.