First things first, go out into your garden and cut some stems off your basil plants. The best time to cut basil is apparently 10am in the morning as this is when the leaves contain the most oil which is what gives basil it's delicious smell and flavor.
For this Instructable I used the entire top half of 4 basil plants, but you can use as much or as little as you want.
Put all the leaves and stems in a clean sink and let them soak for a few minutes. I don't use any pesticides on my plants but there are still usually little bugs on the leaves, dirt, and the occasional spider web you don't particularly want in your dinner ;)
Swish 'em round with your hand a few times for good measure.