If you see a celeb out and about, the easiest way to record an iPap is to go to ipap.me on your iPhone or Android device. Click on 'Add an iPap' (you will be prompted to log in if you haven't yet). Remember to click on 'Allow' when the browser asks if it can use your location. This will use your current location and the current time.
If you want to record a iPap from a couple of days ago, use method 3.
The mobile site will work on your smartphone if it supports current Web Standards, and Javascript. If the mobile site doesn't work for you, don't panic! You can record an iPap by method 3.
Use the map tools on the left to zoom and scroll, click on the spot where you saw your celeb.