To form the comparative, you add the ending “-er” (schön-er). To form the superlative, use the word “am” and add the ending “-sten” (am schön-sten).
cool
clean,clear,cleanest
sauber, sauberer, am saubersten
sauber, sauberer, am saubersten
clean,cleaner,cleanest
pretty, prettier,prettiest
schön,schöner, am schönsten
schön,schöner, am schönsten
pretty, prettier,prettiest