Let's have a look on how to form the comparative and superlative of adverbs:
Adverbs that end with "-ly": add the word more for the comparative form, and most for the superlative form.
For example: He drives slowly - He drives more slowly than John - He drives the most slowly of all the three.
Adverbs that do not end with "-ly" and have the same form as the adjectives: Use the rules described in the comparative and superlative table of the adjectives. (the table can be founde here: Comparative and superlative of adjectives)
For example:
She drives fast - She drives faster than John - She drives the fastest of all the three.