How to Hand Whistle
To hand whistle, start by cupping your left hand like you're holding water in it. Turn your hand so it's vertical with your thumb pressing against the top of your index finger. Do the same with your right hand. Then, cup your right hand around your left hand with both thumbs resting on top. You want your palms together so that there's only a small gap between your thumbs. The key is to create a tight chamber with only one air hole. While holding the shape, purse your lips like you're making an “Ooh” sound and press your lips against the hole between your thumbs at a 45-degree angle. All you need to do now is blow air towards your fingers. Getting the right sound can take a bit of practice, but keep at it and you'll get there!
Cup your left hand like you are about to drink water from it. Water-tight is airtight, and you need to prevent air from escaping your hands to make them whistle. Rotate your hand to the right 90° like pouring the water out. Your left thumb joint should be against your left index finger, just behind its middle joint and your hand cupped to the right, forming a “C” shape. Curl your right hand slightly, as if you were about to give a handshake. Your fingers should be close together and slightly bent. Your thumb will rest on your index finger. Cup your right hand around your left to create a golf-ball sized chamber inside your hands. You want to form an airtight chamber between your hands. To do so: Your left index and middle finger will rest in the space between your right thumb and fingers. Your palms will meet to seal off the back of your hands. Your right fingers will rest on top of your left fingers. Touch your thumbs together so that there is a small opening between the bottom knuckles. You want a small, rounded slit – roughly 2-3 centimeters tall and ½ a centimeter wide – like a small, squinting eye. Purse your lips as if you were making an “ooo” sound. You lips will be puckered out, but somewhat close together. Imagine you are "booing” someone on stage. Place your lips on the top of your thumb opening at a 45° angle. This part often takes some practice. Your thumbs should be pointing away from your lips slightly, with a small bit of the opening exposed near your chin so the air can escape and whistle. Your upper lip will rest near your thumbnail. Blow evenly into the opening. Imagine you are trying to blow out a lot of candles at once. You don't want to spit, blow rapidly, or blow softly. If you've do ne everything correctly, you should hear a clear whistle coming from your hands. Make sure your hands are cupped. When you blow into them, you should feel the air pushing them apart as pressure builds up. If it does not, the air is leaking somewhere. Take a deep breath and then exhale slowly and evenly into your hands. It should take 10-12 seconds to run out of air. Make the space between your thumbs smaller. If your hands are too far apart you will hear a deep, low exhaling sound as you breathe, almost like Darth Vader's breath. You should hear a thin, high-pitched “whoosh” of air even if you don't hear a whistle, so move your thumbs closer together to improve your sound. Adjust your mouth position. While blowing, slowly rotate your hands up and down. Many people put their mouths right up against the hole. You need to have small space open under your bottom lip, and your upper lip should seal any holes between your thumbs. Change the pitch of the note by blowing harder and opening your hands slightly. You can always change the note you are trying to whistle. It all depends on how hard you are blowing and how tightly the back of the hand-cave is closed. If your palms open some, you will get a higher-pitched sound than when they're tightly shut. The more you open, the higher the sound will be. There are, of course, limits to this, as opening too much will prevent any whistling at all. 
Cup your left hand like you are about to drink water from it. Rotate your hand to the right 90° like pouring the water out. Curl your right hand slightly, as if you were about to give a handshake. Cup your right hand around your left to create a golf-ball sized chamber inside your hands. Touch your thumbs together so that there is a small opening between the bottom knuckles. Purse your lips as if you were making an “ooo” sound. Place your lips on the top of your thumb opening at a 45° angle. Blow evenly into the opening. Make sure your hands are cupped. Make the space between your thumbs smaller. Adjust your mouth position. Change the pitch of the note by blowing harder and opening your hands slightly. 