It's basically a game. You get in, you get what you want, you get the hell out. Make small talk with the employees if it makes you feel more comfortable, because the key is to blend in. Don't look suspicious. Cool, calm, and collected. If you can do that, you're basically half way there.
The other half is justifying it. But you're young, you're poor, you're pretty, and for God's sake, you're ******* entitled to it, right?

The most important thing to remember afterwards is to not get attached.
Because you stole it, you didn't work for it. You took what wasn't yours and the universe has every right to take it back. And of course, there is Karma, the ***** that caught it all on tape

So steal it. Steal the top and the cute high heels and the shiny ring and the poor boy's heart.
But don't think for a second that it'll stick around.