Most French adverbs of manner are formed by adding -ment to the feminine form of an adjective, roughly corresponding to English "-ly": lente (feminine of lent, slow) → lentement (slowly), heureuse (feminine of heureux, happy) → heureusement (happily/fortunately).
Adjectives already ending in a vowel in the masculine use that form instead of the feminine: vrai → vraiment (truly), poli → poliment (politely). A few common adverbs are entirely irregular and simply have to be learned as their own word: bon (good) → bien (well), mauvais (bad) → mal (badly) — not bonnement or mauvaisement.