All you need is:
A planter (with no hole in the bottom)
A plastic planter saucer
An empty plastic one-liter bottle
A length of PVC, about 2 feet
A drill and saw
Potting soil and plants
And a helper is always handy:
The tutorial is going to be for a 3 gallon planter, so if you use a different size, adjust everything accordingly.
The saucer should be sturdy, not one of the 50 cent flimsy ones, it will have to hold up a lot of soil. To test the size of the saucer before you buy it, it should fit into the planter and hold itself up about 2 to 4 inches from the bottom of the bottom of the planter. Depending on the size of the saucer, you may need to turn it up-side-down to have enough space for the water.
First, you're going to prepare your plastic bottle. The bottle is going to sit on the bottom of the planter and act as a wicking device to get the water to the soil. Drill lots of holes (around 5/8ths of an inch is good) in the bottom half of the bottle.
Next, cut a hole in the center of your saucer, just large enough for the bottle to fit through.
Then, place the bottle in the hole and place the saucer and bottle into the planter. Press the saucer down as far as it will go. Then mark your bottle just above the rim of the saucer. Take the bottle and saucer back out and cut the top of the bottle off along that line.
Now take your pipe and trace the diameter somewhere closer to the edge of your saucer. Cut a hole in the saucer along those lines. This will be your watering tube.
Place the pipe inside your planter and make sure it's not going to stand above the plants and be all ugly. If it is, cut some of the length off. You only need it to be 2 or 3 inches above the soil line. (In my example, the pipe is going to sit at angle. If yours doesn't fall at an angle when placed back into the planter, you will want to notch the bottom tip or drill holes as in the plastic bottle to ensure that water still flows regularly to the bottom.)
Place the saucer back in the planter. Insert the plastic bottle and the watering pipe.
You will need a drain/overflow hole in the planter. You can either measure or eyeball it, but the drain hole should go through the planter and be just under the base of the saucer. It will keep the soil from holding too much moisture and it will let you know when you are done watering by leaking out the side.
Finally, carefully start filling your planter with potting soil. Use a little scoop or cup. Make sure that the plastic bottle wick is filled first, then load up the pot. Plant your seeds or started plants. The first time, in addition to filling the watering tube until your planter leaks out the side through the overflow hole, you should also water the top like a traditional planter. But for the rest of the season, you'll only need to water through the pipe unless the top gets really dry.
And there you have it. Now you are free to go water your plants and forget about them for a week! Have fun and let me know how it goes if you tackle the project yourself.
Another post with more self-watering planter options here.