1. Purchase two 15 liters plastic buckets with lids. Make sure to purchase food grade buckets only.
2. Arrange a ½” drill machine and use it to perforate the bottom of one of the bucket.
3. Similarly perforate one of the lids. Make sure that perforation density of the bucket and that of the lid should look somewhat like below:

click to enlarge
5.
Drill another ½” hole to the side wall of the second bucket. This hole will be used to attach a tap for draining the drinking water.
6. Purchase a food grade plastic tap. The thread end of the tap should be suitable to fit into the ½” drill hole of the second bucket. 7. Tighten the nut of the tap properly, so that there is no leakage of water from the hole. You can use locktite for making the joint leak proof.
8. Next, you have to create the so-called filter element by using gravel, charcoal, sand and excelsior.
9. Clean all the materials properly. Take the first bucket (which has perforated bottom).