Hard to believe that, with one simple blade of grass, you can become a summertime musician! Well, believe it, my friend. What I'm telling you is the gospel truth!
Here's all you have to do:
Pick a piece of grass (wide-blade St. Augustine works the best) that is as long as your thumb - from the bottom knuckle to the fleshy pad at the top.
Lay the piece of grass flat, along the side of your thumb.
Align your thumbs together, knuckle to knuckle, stretching the piece of grass as if it were a reed. Make it straight and tight between your thumbs.
Then, all you have to do is place your lips in the space between your knuckles and blow! When I did it my summer melody was so so high-pitched Tripod tilted his head in wonder and Cassius, the dog next door, began barking.
Play with the tension 
of your blade of grass. For me, the tighter the tension the higher the pitch. Loose tension made a really comical sound...if you know what I mean.
Either way, get out there and make a joyful summertime noise. Toot! Toot!