Theory:

Adjectives are formed from nounsverbs, and some are formed from other adjectives. There are no simple rules for forming adjectives, but there are common patterns. Suffixes are words used at the end of a word to form a derivative word. By adding such suffixes, many new adjectives can be formed. Suffixes generally added are "y", "en", "ful", "some", "al",  "ous", "less", "able", "less", "ative", "ish", etc.
Example:
 Adjectives formed from nouns:
1. Oil + y = Oily
2. Silk + en = Silken
3. Hope + ful = Hopeful
4. Awe + some = Awesome
 
Adjectives formed from verbs:
1. Talk + ative = talkative
2. Tire + less = tireless
3. Play + ful = playful
 
Adjectives formed from other adjectives:
1. Black + ish = Blackish
2. Sick + ly = Sickly
However, in some cases adding suffixes will not suffice. Dropping the last letter and adding a new letter/letters can also produce new adjectives:        
Example:
1. Envy - y + ious = Envious
2. Study - y + ious  = Studious
3. Nature - e + al = Natural
4. Beauty - y + ful = Beautiful
Is there a rule to follow for precise formation of adjectives? The answer is no. Common patterns may be derived, but a rule cannot be established. 
Take a look at some of the common patterns:
  
Words ending with eRemove "e"; add "al"NatureNatural
Words ending with eRemove "e"; add "y"IceIcy
Words ending with yRemove "y"; add "iful"BountyBountiful
Words ending with yRemove "y"; add "ious"GloryGlorious
Important!
These are only patterns, and there is no thumb rule for the formation of adjectives.