NINTH UNDUE COMICAL BURGER CUTLETS SLITS TURTLE REGRESS SORTS PROPHET SQUATS PUPPY ANGINA DUDS SKUNK TIGHT ODD TERSE COLONEL DIVED SOON AREA DIARIST SKUNK DREAD HASH RENTER DOWDY MAMAS GURGLE PORPOISE UNTRUE MADAM YUCKY
Let's solve this backwards as intended.
Observe that every word ...
... has at least one repeated letter.
Further, we can ...
... replace that letter with another letter to get a new word. For example the N's in NINTH can be turned to F's to get FIFTH. One set of possible new words is:
fifth indie nominal budged curlers elite purple legless aorta crochet equate mummy engine nuns stunt sighs off torso coroner river seen prep dearest sauna treat ease dented lowly memes turtle tortoise entree radar sucks
And indeed, ...
... the replacement letters give us the instructions above, but in fewer words. They read:
FIND REPLACEMENTS FOR REPEATED LETTERS