How To: Self-Watering Seed Starter Pots. Cut a 2L bottle in half. Use a Phillips screwdriver and a hammer to punch a hole in the center of the bottle cap. Cut a length of yarn/string (cotton or poly), about 1'-1.5' long, double it over and tie a loop on one end. Thread the yarn/string through the hole in the bottle cap so the knot is on the inside of the cap. Label your bottle so you remember what you planted. Add potting soil and seeds, and water from the top the first time.