The mini-stick button on the Pro Throttle is simply friction fitted to the post of the mini-stick. Simply pull it off.
Because the button is so small, you will want to find a piece of wire or wood dowel that will allow you to hold the button securely while tooling it.
Once you get the button secured on a "holder", use the cut-off wheel to cut the "bulb" off the end of the button. Make your cut right at the edge of the portion of the button where it has the largest diameter. The result will be a very flat button surface.
Next, using a grinding bit, create a concave "cone" on the surface of the button by slowly rotating the button around as you hold the grinding wheel at a slight angle. Position the grinding wheel so that the front edge of it is right at the center of the button. As you rotate the button around, it will create the cone shape. Don't try to take out too much material as you go, just rotate the button as many times as necessary to obtain a nice cone shape.
As a nice finishing touch, add a small film of hot-glue so that your thumb can get a better grip on the button.
That's it! Just press the button back on the mini-stick shaft and you're ready to go.
Though a very simple and quick mod, you will never want to use the stock mini-stick button again once you lay your thumb in this bad boy!