Flatten the bottom and leave a bit of a tenon large enough to accommodate the spur center in a moment. While it gives lots of meat for the spur center, it also gives a definite end to the bowl for design purposes so as to later prevent turning the inner bottom too deeply.
At this point I flip the piece headstock to tailstock and procede to rough out the center. In other words, the point of the spur enters the indentation left by the tail center and vice versa. This is one of those times it is good to remind yourself that "this is just a bowl." Rough it out like you would any other. The difference is that the surface will not be flattened because it does not exist as a flat plane. A great deal of what we turn is air. Look for the ghost and turn it away. You will be able to see the tip of the tool as it cuts away the inner part of the rim. Shavings will fly out of the side of the bowl. Take a look at the pictures and see how it goes. Leave the sides and bottom about 3/4" thick or more. It is hard to say to leave it 10% of the diameter because there is so much variation in diameters around visible edges.
Part way in the tool is contacting far more of the wood and at the center as opposed to the edges is ow cutting in a continual circle.
Remember to leave a good sized center tenon in the bowl as the turning proceeds. This gives needed support to the possible imbalance of the shape of the natural edged turning.
Once the sides are all even in the rough out, taper the tenon to the bottom, getting ready to remove it. Do not make it too narrow as it will still be needed for support in removing the other tendon.
Once again flip the bowl over and re-establish the centers. Turn away as much of the bottom tenon as possible, leaving a small nubbin.