You can't, and that is by design.
That's the short answer. The longer answer is that it's possible, but it's a violation of the Windows EULA so it's illegal, it's extremely difficult to do even for people who know what they're doing, which you don't or you wouldn't even need to ask, and the end result is really not very useful. So you have one legal, and two serious technical issues to contend with. Which is why I will now refer you back to the short answer.
How do I create a Live USB/CD of Windows 7?