Are you intending to build the entire interface board from raw components? An in-between alternative would be to buy a pre-built USB to SATA controller board, and then build your own 12V/5V power supply for it.
If you're going with bare components, you would presumably need to map the pinout of the controller chip to a SATA connector on one side and a USB connector on the other, plus supply it with necessary power (probably 5V, which may come directly from the USB port even if the drive isn't bus-powered--I have enclosures that work that way); the pinouts on the chip should have either come with it or be available from the manufacturer. Past that, it should be just a matter of power circuitry to supply the controller chip with whatever power it needs, and the drive with 12V/5V power via a SATA power connector, the pinouts of which are easy to find online.
You should probably also put a voltage regulator in there, in case the power brick is out of spec--I had a cheap slimline external enclosure that required 5V in and had no regulation at all, so when I accidentally grabbed the wrong brick and gave it 7.5V, it fried an $80 blu-ray drive (the $10 controller itself, being bus-powered, ironically survived).