how do you make crossfeed actually work (trust me this is actually a good question)

I've always been fuzzy on this as well. The way I've made it work with my Astrum Heavy is by saving the side interstages of the "Simple Heavy". Those seem to be XML edited to not have the fuel cross feed setting which I'm pretty sure automatically makes it act like it should. And could be wrong but I think you have to make sure whatever is attached to it doesn't have it's fuel line active

Have you tried setting the priority of the tanks?
Combine that with fuel lines and you will get some neat set ups.
Also you can connect your engines directly to your fuel tanks no matter the distance by using the Parts Connection Tool. That will minimize how many parts need fuel lines.
Unless the engine needs to move separate from the tanks, that will work.
Another thing, are you trying to fuel up with in the same craft? Im not sure if that works. You might have to place a command chip on those tanks and separate them with a docking port to get it to by pass

Decouplers can be used to control the direction of fuel flow across fuel tanks. Decoupler fuel crossfeed option very easy to find.

If you want multiple fuel tanks to act as one single fuel tank, you can use the 'Fuel Line' option in the 'Advanced Part Settings'.