You can do either. Some people will claim that the best hash to be had is made fom gathering the trichomes off of the live plants, not the dead and dried plants. Using bags and ice extraction is one method of making hash, there are numerous alternative methods which will make worse, comparable, or superior products. Hash is trichomes, that is pretty much it, there is no "propper" method to making it, if you remove the plant material and get left with the trichomes, then you have suceeded, and done a propper job, regardless of the method used.