There's a chapter in Davvin 4 devoted to forming the comparative and superlative of adjectives but very little on adverbs anywhere apart from a note showing how to identify them by virtue of their ending -t. The summary on Oahpa! doesn't have much information either, unfortunately and I can't quite see how it's done on the strength of their two examples.
Oahpa! - Manner Adverbs
Adverbs can also be derived from comparative and superlative adjectives in the same manner:
Dus lea 'čábbát' čála go mus. Don čálát 'čábbábut' go mun.
(You have nicer handwriting than I do. You write more nicely than I do.)
Ánte lea 'jođáneamos'. Ánte bargá jođánepmosit.
(Ánte is the quickest. Ánte works the most quickly.)
I would appreciate very much an informed explanation.