To form an adverb, simply take the feminine form of an adjective and add -ment to the end.  If the masculine form ends in -e, you just add the -ment to that.  Adjectives ending in -ent or -ant take the endings -emment and -amment.

Note:  Some adverbs such as actuellement (currently, now) and éventuellement (possibly, perhaps) can be deceiving.  

Note:  Adverbs are placed right after the verb in a simple tense.  Adverbs of opinion and time usually go at the beginning or end of the sentence.  When peut-être and sans doute begin a sentence or clause, they are usually followed by que.  With the passé composé, most adverbs are placed between the auxiliary verb and past participle.  In negative sentences, pas precedes the adverb, except with peut-être, sans doute, sûrement, and probablement.  Adverbs of time and place generally follow the past participle.