This rule is about forming an adverb from an adjective, not about using an adjective in a sentence.

What this rule says is that to form an adverb from an adjective, you take the (singular) feminine form of the adjective, and add the suffix “-ment”. Examples:

certain → certaine → certainement
lent → lente → lentement
doux → douce → doucement
long → longue → longuement
actuel → actuelle → actuellement
actif → active → activement
financier → financière → financièrement
affreux → affreuse → affreusement

This rule is productive: you can take an existing adjective that has no corresponding adverb, and apply this rule to make a new adverb.

Beware that as with all etymological rules, there are exceptions.

Sometimes the -e- is pronounced with a semi-open sound [e] and takes an acute accent accordingly.

confus → confusément
immense → immensément

Adverbs formed from a past participle don’t take an extra -e-:

modérer → modéré → modérément
étourdir → étourdi → étourdiment
devoir → dû → dûment

More generally, adverbs from an adjective that ends in a vowel usually don’t take an extra -e- (but keep the existing -e if the adjective already ends with -e).

aisé → aisément
joli → joliment
vrai → vraiment
absolu → absolument

Exception to the exception: gai → gaiement.

Adjectives that end in -ent or -ant lead to an adverb ending in -emment or -ammant (pronounced [amɑ̃] even when it’s spelled -emment). There are a few exceptions (lentement, présentement) but the productive form is -emment or -ammant.

courant → couramment
différent → différemment
savant → savamment

And some other exceptions that evolved on their own:

gentil → gentiment