Don't take it at all. There is a long list of unproven indications for kava. The most common reason I see it used for is for stress reduction. I do not recommend the use of kava because it is known to be a liver toxin even after short term use and it interaction with a number of medications which could make them ineffective and/or more toxic because of the high level that your body cannot effectively break down.
Standardized extract. If you are going to use kava it is wise to find a standardized full-spectrum extract. Objective information is hard to find but search kava extracts and read user reviews. Start with a low dose and experiment to see what dose is best for you. Here are a couple good sources: http://kavasseur.blogspot.com/2011/04/paradise-kavas-answer-to-kava-extract.html & http://tinyurl.com/qeckbdm.