Refurbished Apple iPad's - Siri User Guide Tips, tricks and tutorials on using Siri on the iPhone, iPad and Apple Watch - buyEtail.com

The first thing that confuses even iPhone power users is that there isn’t an Application icon for Siri. So now that you’ve set up Siri how do you start?

[box]UPDATE: As of iOS8 there is a new handsfree mode when you’re plugged into a power source.[/box]

It’s actually really simple:

1. Press and hold the iPhone’s physical “Home” button to open Siri. (Or hold it up to your ear if you have enabled the ‘Raise to Speak’ option).
2. You will hear two quick beeps to tell you that Siri has woken up and is waiting to do your bidding.
3. Once Siri has opened,  ask a question or ask Siri to perform a task such as emailing or texting. Siri will automatically detect when you have stopped speaking, but you can optionally press the microphone icon to show you have finished speaking.
4. If you requested Siri to do something such as send an email or text, Siri will show you a draft of the proposed message before it is sent.  To send the message,  say “send” or touch the “send” button.

You can generally speak in a natural way when asking for help from Siri. 

source: http://www.siriuserguide.com/how-to-use-siri/