There are a couple of different ways you can take a selfie using the Hipstamatic app.
The first works well if you're using Classic Mode. To use that method, double tap the smallviewfinder on the Hipstamatic app. You'll find it becomes fullscreen. (If you're using the camera in the regular way, that helps you frame and compose your shots.) If you want to use the camera to take a self-portrait, just flip your phone around and tap anywhere on the fullscreen viewfinder to take a pic. Because the iPhone camera is slightly wide angle, you're very likely to capture yourself in the frame without too much trouble.
In Pro Mode, there's another option. You can access the front-facing camera by going into Pro Mode in Hipstamatic and using the camera case icon near the upper right corner to switch between the two cameras.
If you have any other questions or concerns about the app, please don't hesitate to get in touch at email@example.com.