Requesting feedback is easy. On the 'Instant Feedback' tab, simply click 'request feedback' on the top right side. You can ask for feedback from multiple persons at a time - simply add their names in the corresponding form. You can also add specific skills you would like to get feedback on. Adding a note makes your request more specific and usually triggers more helpful feedback.
Furthermore, you can request feedback by clicking on a person's profile on your home screen and then 'request feedback'.