To prepare a stock solution of ethidium bromide, add 10g of ethidium bromide to 800 ml of distilled water in a suitable container. Add some more distilled water to the container until the volume reaches 1 liter.
Stir this mixture on a magnetic stirrer until the dye is completely dissolved. This could take several hours. When the EtBr is completely dissolved, wrap the container in aluminum foil and store at room temperature. Alternately, transfer the solution to a dark bottle and store at room temperature.