Start at the Instrument Driver Network and at the bottom you will see a section 'Resources for Instrument Driver Users and Developers'. Lots of links on the 'Learn to Build Instrument Drivers' tab.
If you don't need a full driver, you could also use the Instrument I/O Assistant. A driver will be something that is more flexible but you can probably get started using the instrument faster with the assistant. Power supplies are some of the easiest instruments to write code for since they typically have so few functions. You can certainly look at drivers for power supplies from other vendors to get an idea of the general architecture.
