To make a rosette might seem as an incredibly time consuming and difficult task; of course with time and experience one can get proficient at it, but even still it is very time consuming. Here's an explanation of the basic reasons for doing that. Now, how do you actually make it?
The first step is to come up with a design; the example that follows is of a fairly classic style, composed of a central mosaic and outer and inner bands of purflings. For the mosaic, you have to use a grid, either on paper or in a computer program; fill the grid with the pattern of your choice. Then complete the design with bands of colored lines that represent the purflings; i quite prefer a symmetrical arrangement for that. That's what i came up with for this specific rosette.

In this mosaic, there are 7 different rows that alternate to give the repeating pattern. Now the actual work can begin. Each color represents a kind of veneer (3 different kinds in this example).

All the veneer have to be exactly the same thickness (0,6 mm for this). First you need to make 7 blocks (stacks) of veneer following the scheme.

Once you have the blocks, you need to cut strips along the grain roughly 1mm thick.

This strips will need to be stacked in the proper order to form the planned design; two problems araise at this stage: first, since the height of every single mosaic square is already fixed by the veneer thickness, you have to carefully thickness every strip to the same dimension as the veneer (0,6mm) in order for the mosaic to look as it is made by square blocks; second, the mosaic will not be mounted in a straight line, but on a circle instead, so you better thickness the strips thinner on the 'bottom' side and thicker on the 'top' side. You can see that this way the blocks will not be square (or just a few of them), but life's not perfect... I prefer to stay on the thick side, so the mosaic will look slightly wider. If you are confused and don't understand what i'm talking about, don't worry, just keep reading.
This is before and after thicknessing.

When all the strips are ready, you can glue them together to form a mosaic 'stick'; i make this with 13 strips in this example, enough for a whole repeating pattern. These are glued in a pipe section, about the diameter of the finished mosaic in place.

Once glued, you should check top and bottom width of the stick and make necessary adjustments.

Next, the sticks are cut in slices slightly thicker than the finished rosette (about 2mm). A table saw with a fine blade helps quite a bit.

Now you need to cut strips of veneer that will make the outer and inner bands, and they should be closely as wide as the mosaic's thickness (2mm).

Arrange them in the planned sequence.

You could now cut a groove in the top and assemble the whole rosette in it; i like much more to assemble it in a groove cut in a piece of metacrilate (plastic); when it's dry you can easily get it out. That gives me more time to assemble it and when i glue it into the top, the process is much quicker avoiding to get too much water in the wood, that could cause the top to warp.
You just start on one end, putting a mosaic tile between the two groups of purflings; get a few centimeters of the purflings wet with hot hide glue, add another tile and one by one assemble the whole thing, adding glue every couple of tiles or so as you proceed. The circle does not need to be closed, because the fingerboard covers quite a bit of the rosette.

That's about it! Now sand the exposed face flat (when it's dry!), so it can glue perfectly in the groove you'll cut in the top, and glue it in there.