We suggest to start with 5-8 crystals in a sachet bag. You can adjust your level of fragrance by how many pieces you use in the bag. Place the sachet somewhere with lots of heat and good airflow. Some of the best places to put them are on the dashboard, in an OPEN ashtray, hung from the rear view mirror or hung from the blinker shiftier. You do not want to place the bag under the seat. There is not enough heat or airflow to activate the fragrance. Once the fragrance starts to weaken, shake and break them just like you would if using them on a warmer. DO NOT PLACE ON LEATHER OR WOOD SURFACES! IT WILL RUIN BOTH! The salt will last you about a month in the car. It will last a little longer in the winter and a little less in the summer.