Say how things happen: rápidamente, bien, despacio
Take the feminine form of the adjective and add -mente — like English -ly.
rápido → rápida → rápidamente (quickly)
lento → lenta → lentamente (slowly)
fácil → fácilmente (easily) — no separate feminine, just add -mente
If the adjective has an accent, the adverb keeps it: rápidamente, fácilmente.
The two most common manner adverbs are irregular: bien (well) and mal (badly). Never "buenamente" for this meaning.
Cocinas muy bien. (You cook very well.)
Dormí mal anoche. (I slept badly last night.)
Watch out: bueno/malo are adjectives (describe nouns); bien/mal are adverbs (describe verbs).
Some everyday manner adverbs are single words with no -mente.
despacio (slowly): Habla más despacio, por favor.
deprisa (quickly), alto (loudly), bajo (quietly), claro (clearly)
When two -mente adverbs are joined by y/o, only the last one keeps -mente.
Habla lenta y claramente. (He speaks slowly and clearly.)
Manner adverbs usually follow the verb (or the whole verb phrase).
Conduce con cuidado / cuidadosamente. (He drives carefully.)
Explicó el problema claramente. (She explained the problem clearly.)
Traps for English speakers
These are the errors English speakers make most often.