Empty the plastic jug. It is best if you use a container that held water. If you are using an old milk jug, clean it out thoroughly with vinegar or a 10-percent solution of diluted bleach. Scrub it with a bottle brush and rinse thoroughly before using.
Pierce a small hole in the bottom of the jug. Don't make it too large to start out. You can always make it larger later.
Place a shepherd's hook or bracket over your birdbath.
Fill the jug with a small amount of water., replace the lid and hang it on the hook.
Test the drip rate. You want the water to drip at a rate of about 10 drips per minute. If it is not dripping fast enough, you can enlarge the hole. Tightening and loosening the cap will also adjust the flow rate.
Fill the jug up with water once you have an acceptable flow rate. Place your finger over the hole to keep the water from dripping out as you replace the jug on the hook. Refill the jug as necessary.
Clean the jug regularly with vinegar or diluted bleach to prevent any mildew, fungi or bacteria.