As has been said, bone dry. My missus used to tumble dry my scrims then take them out and fold them up. She thought they were dry, and they were, but they wern't bone dry. Always make sure the part of the scrim you use is as dry as dry can be, otherwise you'll be leaving moisture on the glass with crud in it.
And one finger in the scrim, otherwise if its bunched it bridges across from the frame to the glass leaving a void that doesnt get scrimmed, thus doesnt get cleaned.
Matt