Most end in 'ly' which is equivalent to the ending 'mind' n Castilian. They generally derive from adjectives

Adjetive	play Adverb
slow (slóu)	slowly (slóuli)
easy (iisi)	easily (íisili)
careful (kérful)	carefully (kérfuli)
simple (símpl)	simply (símpli)
happy (jápi)	happily (jápili)
natural (náchural)	naturally (náchurali)

     

There are also exceptions to this rule:

Adjetive	play Adverb
good (guud)	well (uél)
fast (fast)	fast (fast)
hard (járd)	hard (járd)

  
More examples with adverbs and their opposites:

Adverb	play opposite adverb
well (uél)	badly (bádli)
better (bérer)	worse (uérs)
together (tchuguéder)	separately (sépritli)
carefully (kérfuli)	carelessly (kérlesli)
right (ráit)	wrong (rong)