code is not correct
1,
<?php if (function_exists ('ba_hook') { ba_hook ('my_custom_hook'); } ?>
2,
my_custom_hook must be replaced by name of hook you've defined... i.e. home_banner, search_banner... depends what you created.

I can not figure out in which folder should I go to put the code and in what position do I have to go to the code <? php if (function_exists ('ba_hook') {ba_hook ('my_custom_hook');}?>

@Fabio
Do it as anyone else - trial and error.
Start on top, check position, move to bottom.
Or you can use browser console to find correct line you want to put banner.