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How can I teach adjectives to beginners and upper intermediate students?

Adjectives are describing words- that's the easiest explanation for this level. eg I have a pretty dress. There is a black cat etc. Look at the post below, there are lots of websites and tips for you. Good luck.
http://www.italki.com/knowledge/contribution-ENGLISH+GRAMMAR.htm

you cannot just decide to teach adjectives.

there is a stuctured and ordered way of teaching english, which includes everything about adjectives

Yes, you should teach adjectives in connection to suitable nouns. And situations.

I suggest adding opposite adjectives while you're doing this. And alternatives. Or a whole spectrum of graduations (/grades /increments):

pretty dress - ugly dress
'ugly' is quite straight. It would be more polite to use 'unattractive'.

worst - worse - bad - good - better - best
(You can try to put in more nuances...)

Hello Jane,
For beginners the picture association is a very good idea as mentioned before. Associating the words with pictures is a good way to memorize them when introduced to them for the first time.

At an intermediate level they could be pointed out in comprehension texts and explained then practiced through exercises related to the text.
Studying the adjectives in pairs: the adjective and its contrary is in all cases a useful method to acquire as many adjectives as possible in the most efficient way.