The standard Hand Whistling technique consists of cupping your hands together, making a chamber, and leaving one hole for you to blow through, and perhaps more. The air will resonate in the chamber and escape producing a whistling sound. The pitch is altered by changing the shape/size of the chamber and/or altering/creating holes for the air to escape through. When it comes right down to it, the ability to Hand Whistle comes from an indefinite amount of time spent cupping your hands and blowing until you finally hit a note. That's not to say that online guides or an actual professional can't help tremendously; they can and will, but Hand Whistling won't come easily. You will have to try again and again, until you can somewhat reliably make a noise. When you've done that, good, but you're still quite a ways a way from being a professional Hand Whistler. You'll also have to increase your range, improve your tone, and eventually even learn to match pitch.