My understanding is that the device used to actually fire the uh...fire gets referred to as a "Siphon," which is essentially a 'tank' of the fuel, attached to a bellows to provide air pressure, with a tube to launch it. There's an illustration in the Osprey "Byzantine Naval Forces 1261-1461" of a hand-held siphon if that helps! And illustrations in manuscripts like the Madrid Skylitzes show the larger naval version attached to the fronts of ships
There's a description in the Alexiad of Greek Fire being used from the walls of Dyracchion to burn the Norman siege engines in 1108 as well, along with a (possibly anecdotal) description for how the Byzantines prepared it:
https://en.wikisource.org/wiki/The_Alexiad/Book_XIII#Chapter_IIII'd assume most of the time it was confined to defensive sieges and naval battles though, just because dragging the equipment around would be pretty impractical in a fast-moving battle, especially where you can't reliably assume wind direction!
The Wikipedia article Mick_in_Switzerland linked has a lot of the main sources about it mentioned, with some contemporary depictions, so it's definitely a good start if you're doing some research