The key to placing comic relief is timing. You could have a character make a funny comment or have a comical misunderstanding happen just when the plot seems intense. Another way is to have a humorous subplot running parallel to the main story that pops up now and then.
Well, to place comic relief, you need to understand the tone of your story. Insert brief humorous interludes when things get too serious. Also, use unexpected twists or quirky dialogues for that comic effect.