Take an empty pop bottle - I have used a 2 litre water bottle:
Yours doesn't have to be the same:
Also make a larger hole in the base of the bottle. Make this about 3" up from the bottom as this will be your hole to fill the water-well.
Next, slip the top half of the bottle inside the base of the bottle. With the neck pointing down. You might need to use a bit of force to get it to fit inside. Make sure you wedge it in tight and you should have something that looks like this:
Now test it out!
Make sure when you're filling it to pack the compost tight into the neck hole of the bottle. This stops the water getting into the compost easily and turning it into sludgy muck.
Your plant will go days without the need for watering, no need to feel the soil - Just check the water-well. If it's running low, top it up! Hey presto.... A self-watering pot!