To be more specific, the ftw register is only used for RAM profiles, and not the simple single tone output. Essentially, just remember that you need to use set(), and don't worry about the other functions unless you are using more advanced features, for which you'll need to understand the AD9910 in a fair bit of depth.
One other gotcha to be aware of is that if you do set(amplitude=XXX) without setting a frequency, nothing will come out because it will actually set the frequency to 0 Hz, and likewise set(frequency = XXX) will overwrite the previous amplitude and set the output to full power
