Well, if you want details, make sure you grasp a couple of things:
Most buck/boost transformers have dual-voltage primaries and secondaries, so you must make sure each set is connected in parallel (low v) or in series (hi v).
The primary is easy, because it's always wired for the supply voltage range. You wire them in parallel for 120v and in series for either 208v to 240v or 240v to 208v.
The secondary can be either parallel series, depending on the supply voltage (actually measured is best), the desired voltage, and whether you're boosting or bucking.
Every BB I've seen comes with a chart of voltages and loads, which point to a diagram or chart that shows how to connect everything for the desired voltage output.
Just to add, you're merely supplying the primary with the circuit supplying the load. The BB is being connected as an auto-transformer, which is a single winding with taps.