Does the word exist out of
 one syllable (lettergreep)?
Yes --> then you use (+er) for the comparative (vergrotende trap) and (+est) for the superlative (overtreffende trap)
Examples: (easy) easier than / the easiest, (big) bigger than, the biggest.
If the adjective (bijvoeglijk naamw) has more than one syllable. 
Then you use (usually): more for comparative (vergrotende trap)
and the most for the superlative (overtreffende trap)
Examples: more beautiful than, the most  beautiful.