Generally, it makes sense to apply a spell's effects in the order they are listed in the spell's description, unless otherwise specified by the spell.
Divine Word says:
You utter a divine word, imbued with the power that shaped the world at the dawn of creation. Choose any number of creatures you can see within range. Each creature that can hear you must make a Charisma saving throw. On a failed save, a creature suffers an effect based on its current hit points:
50 hit points or fewer: deafened for 1 minute
40 hit points or fewer: deafened and blinded for 10 minutes
30 hit points or fewer: blinded, deafened, and stunned for 1 hour
20 hit points or fewer: killed instantly
Regardless of its current hit points, a celestial, an elemental, a fey, or a fiend that fails its save is forced back to its plane of origin (if it isn’t there already) and can’t return to your current plane for 24 hours by any means short of a wish spell.
If Divine Word affects a fiend with 20 or fewer HP, it should be immediately slain, then banished to its home plane (assuming that slaying it doesn't do that already, like it would for a summoned creature from a spell like Conjure Fiend). The other effects of the spell would be applied in order, but because they are redundant, you can usually just skip to the bottom-most effect. If there were a creature that had the ability to come back to life after being slain, tracking the other effects would be necessary, however, as the creature would still be affected by them until the duration expired, since dying wouldn't technically remove them.
Alternative Interpretation: The effects happen simultaneously
In the event that multiple effects happen simultaneously, Xanathar's Guide to Everything includes the optional rule that the controller of the creature suffering the effect chooses the order in which they are applied. So in this case, since the DM controls the target, they would choose whether the creature is slain or banished first.