On the top-center of the corm, surrounded by the leaf bases, is the apical meristem located. The basal tissue is the corm tissue itself. Starting from a potted plant, you have to cut the pseudostem, roots, and clean the corm all around. You may end with something like this:
That would be small enough to start disinfection, and then you need to go on cleaning again to remove more leaf bases and corm tissue to reach an approximate size of 1 cm3. Often people just cut that 1-cm3 cube in two halves in order to have 2 explants from every apex. The reason is that there are axillary buds at the insertion site of every single leaf base, so by adding cytokinins you activate them and therefore you dont worry too much about the apical meristem. For your first time I would not recommend to do this. Instead, keep the piece as a whole and add cytokinins. You would probably get a main shoot and induce lateral shoots at the same time.
Check this for a halved explant:
Tissue culture musa velutina, musa zebrina and 1000 fingers
Good luck and keep briefing ...