Go to zoegriffin.ipap.me on your iPhone or Android phone. Click on 'Add an iPap' (you will be prompted to log in if you haven't yet). Click on 'Allow' when the browser asks if it can use your location.
Your iPap will be recorded using your current location and the current time. Easy peasy!
Zoom and scroll using the map controls on the left, and simply click on the spot where you saw your celeb.
You can tell us when you spotted them and give us the gossip on what they were up to!