Scan, help!

Hey all, first timer here :) we have our 12 week scan in the morning and just a bit of advice needed please. The letter says i should drink a pint of water every hour up to the scan and hold it. This isnt very clear to me,...?!
Also my appoinent is at 9am so dont really know when to start drinking? 
How much water did you ladies drink beforehand and when did you start? Xx