You will need:
1. Cut the top off the PET bottle using scissors or a sharp knife.
2. Add a layer (about 1 cm deep) of charcoal, followed by a layer of pebbles (about 2 cm deep) and a layer of soil (about 5 cm deep).
3. Stick some holes in the soil and add some small plants. You could also add a layer of moss. Tape the top of the bottle back on.
4. Place your mini biosphere in direct sunlight and water occasionally but not too often - the water in the container should evaporate and condense on the sides of the container, creating its own water cycle.
...you just need to add sunlight and some water. The charcoal acts as a filter, purifying the water as it cycles through the system. You can also create an aquarium in a PET bottle, with water plants, rocks and some water from your local creek.