First, you will need .Z80-format snapshots of the Spectrum games you want to play. A good place to look is World Of Spectrum.

At present, Foon only supports .Z80 format snapshots. If the game you want to play is not in .Z80 format, you can use the ZX-32 PC Spectrum emulator, load the game, and save it out as a .Z80 image.

Next run the executable, and drag your .Z80 files from Explorer into the large white area at the top of the dialog box. You will see games being added to the list. Double-click on an entry to edit the key mappings for that game.

You can map any GBA button except Start to any Spectrum key or Kempston Joystick motion.

The defaults for each game are basically Kempston joystick motions. If the game requires you to, for instance, Press 0 to Start, then don't forget to assign one of the spare buttons to "0". There will eventually be support for dynamically generating unassigned keypresses at runtime, but not today.

Once you've assigned all the appropriate keys, select an output file, and hit the Create Image button. This will generate a .BIN ROM image file that you can transfer to your GBA flash cart. Note that the Injector doesn't fix up the GBA rom header, so you certainly get a warning from the Flash Advance Writer software. Ignore it.

Put the cart in the GBA, switch on, and you should be up and running. Use the Start button to cycle through games.