CO2 extraction is the best method to extract pure CBD isolate, in which state it exists as a clear/white crystal. This method is preferred over other methods because it leaves practically zero traces of chemical contaminants in the end product.
Alcohol extraction is another method, wherein a liquid such as ethanol, butane, or hexane is run through raw plant material to extract the CBD and then evaporated to leave only the active cannabinoid. This can be a dangerous method because it’s prone to leaving excess traces of the butane or hexane chemicals in the extract.
Also, infusing vegetable oil (i.e. olive oil) is another effective way of extracting CBD. All you do is grind up the raw plant material, decarboxylate (heat) it, and let it sit in boiling (100-deg Celsius) oil for about 2 hours.