If you are a student:
Download this document and look at the words in the left columns. Write, in the right columns, the words with the opposite meaning of each given word.
Then download this answer sheet and check your answers.
If you are a teacher:
You could have your students do it like explained above but there is a fun way of doing this activity in class.
I actually have all the words on the antonyms exercise written on different coloured papers. I stick all the papers in a random order on the board. Then I divide the class in half. Both teams get their own marker colour. Then one person from each team comes to the board and they both write what they think the opposite is (have the students write on each side of the paper, without looking at what the other is writing). Repeat until all the words have been done.
It is a lot of fun this way because it adds a little bit more competition and the students tend to remember the words they did not know better.