(Credit for this guide goes to JohnnyStrong from the forums)
The function keys can be assigned to certain powers, and pressing a function key will assign its power to the RMB (right-mouse button).
Function keys are assigned in the Powers panel. Hover the cursor over the power, and then press the function key that you want assigned. From that point onward, pressing that function key will cause RMB to use that power.
For example, playing Cyclops involves a handful of different beam attacks. Assign each of these attacks to a function key, and then you can quickly change the power that your RMB uses.
In addition, in the keyboard options you can re-assign the function keys to other keys. In my case, I had done the following:
Prior to using the function keys, I had reassigned GH to WE, which I found easier to reach than GH. So with respect to the function keys, I assigned F1-4 to ZXCV, F5 to Q, and F6 to R.
Using this configuration meant that my left hand could quickly access the available hot keys, and it was just a matter of practicing where each power had been assigned. I might refine this arrangement, and you might have your own preferences, but I'm mainly trying to explain how the function keys work.
Note that W is a default hotkey for moving forward, which can also be reassigned. After practicing with the default W move forward, I decided to remap keys as follows:
Assign '-' and '+' for minimap zoom out/in.
Assign open map from [Tab] to M.
Assign move forward (W) to [Tab].
You can also bind function keys to LMB.
To do this you must click on the LMB power icon (far left on hotbar) and a list of available skills will pop up above the icon. Once the pop up icons are visible you can hover over them and assign a function key just like the normal method.
This method works for both LMB and RMB but the Powers window method only works for RMB.