For the written how-to, visit http://www.phonebuff.com/2012/04/copy-paste-iphone-ipad/
In this video, I show you how to use the copy and paste feature on the iPhone and iPad. Copying and pasting can be useful for when you are trying to share a sentence or two from an article you've read (without having to send the entire article), if you're trying to copy a URL from an e-mail to Safari, if you want to research something you read in more depth, etc.
To activate the feature, simply press and hold on a word within the sentence you're trying to copy. Then, use the blue sliders to highlight the exact text you want. After you let go, the "copy" button should appear. Just press it and the text will be stored onto your clipboard. Then, go over to where you want to paste the text, press and hold within a text field, and then hit the paste button that pops up.