If you look at the schematic, you'll see that the feedback pin is tied directly to the flyback output voltage. In the schematic you attached, that would be a 12V version of the part. The control loop inside the chip will adjust the switching time/rate to keep the feedback pin at +12V. If you got a 3.3V version, then the chip would work to keep the feedback pin at 3.3V, instead.
Adjustable just means that the feedback pin will try to stay at some voltage (per the spec sheet, Vref is 1.23V). In this case, you would need to set up a simple resistive voltage divider to take your desired output voltage, and divide it down to 1.23V. If you wanted to have Vout = 8V, then you could use R1=6770 and R2=1230 ohms (or something close). The voltage divider would only steal 1 mA of output current to work correctly.
This device and schematic says it can handle +12V and -12V at 150 mA, so it should be more than capable of supplying your small current needs (around 2mA).
The easiest way to learn is to find a diagram, buy some parts, and start putting something together. I'm not a power converter designer, but I know the theory and general concepts well enough to get me by. I'm sorry, but I don't have the time to walk you through a design. That is something that takes several hours of time, even for an experienced designer.
When I was an undergrad, I started with a Simple Switcher and the basic schematic and made my first flyback, so it should be a pretty easy place to get started for about anyone.
Good luck!