To form the feminine plural, just add an -s, unless it already ends in an s, then add nothing.  To form the masculine plural, just add an -s, except in these cases:  -al becomes -aux (exceptions: banal - banals, and final - finals); -eau adds an -x; and if it ends in an x or s already, add nothing.  Just remember to change the le, la, or l' to les.  

And of course there are more exceptions... some adjectives are invariable and do not have femine or plural forms.  Compound adjectives, such as bleu clair (light blue) and vert foncé (dark green), adjectives that are also nouns, such as or (gold) and argent (silver), and the words chic (stylish), bon marché or meilleur marché (inexpensive) never change.