Spray the bottom of the 18-inch terra cotta saucer with acrylic spray to prevent leakage.
Use an electric drill to drill a hole into the 18-inch saucer for the pump power cord.
Make water-flow openings in the rim of the 8-inch terra cotta pot using a hacksaw. To make an opening, make two notches 1/2 inch apart. Carefully saw a curved shape between the notches until the bit of pot in between cuts or falls off. Make another opening directly across from the first. Create the third opening in the space between those two, and the last directly across from the third. You will create a total of four openings.
Choose a spot on the 8-inch pot to create one more small notch that's large enough to hold the power cord of the aquarium pump.
Attach the 3/8-inch tubing to the pump according to the instructions included with the pump.
Place the aquarium pump onto the 18-inch saucer. Place the 8-inch pot upside down directly on top of the pump, running the pump cord through the notch you made for it on the pot. Run the power cord through the hole you made on the 18-inch saucer, then fill the area around it with sealant to prevent water leakage.
Run the plastic tubing from the pump through the hole on top of the 8-inch pot and cut off any excess tubing (the tube should emerge just above the hole in the pot).
Create a hole in the 8-inch saucer using an electric drill with a masonry bit.
Make an opening in the 8-inch saucer with a hacksaw, using the same procedure you used to make openings on the 8-inch pot.
Run a line of waterproof glue along the edge of the overturned 8-inch pot (the glue should be on what used to be the bottom of the pot).
Run the plastic tubing from the pot through the hole on the 8-inch saucer, and place the saucer on top of the pot with the glue. (This helps the saucer stay on when you're running the fountain and during fountain cleaning and maintenance.)
Place the 4-inch pot, overturned, next to the 8-inch pot. Ensure that your 4-inch pot is near the opening you cut into the 8-inch saucer so that the water flows from that saucer down to the 4-inch pot.
Run a line of glue along the outer edge of the 4-inch pot, just as you did on the 8-inch pot. Place the 4-inch saucer on the overturned pot with the glue.
Place the 6-inch pot with the plant into an empty space of the 18-inch saucer (this is just for decoration).
Fill the top of the 8-inch saucer and 4-inch saucer with a thin layer of river rock or seashells, using the rocks or shells to hold the plastic tubing in place on the 8-inch pot. Fill the empty spaces of the 18-inch saucer with river rock or seashells.
Fill a pitcher with water and gently pour the water into the 18-inch saucer. Plug the fountain in and make sure it runs properly before placing it in its permanent location.