Take an empty milk jug and rinse it out with water. Refill it half way and add a couple of drops of dish detergent. Place the cap back on the milk carton and shake it for a few seconds. Pour out sudsy water and rinse the jug.
Take your serrated kitchen knife and poke it through the milk jug about half way up. Saw around, avoiding the jug's handle. Discard the bottom half of the jug. The top half, with the handle intact, will be used as the spout. Fluid poured into the wide-open end of the jug will funnel through the mouth of the jug.
Using your kitchen shears, trim the ragged edge of the top half of the milk jug so you'll have a nice, even edge.
Save the cap to the milk jug in case you need your spout to hold fertilizer, water, kitty litter or anything else you are working with that you might have to transport to a different location.