Add one teaspoon of water flavouring to 450mls cold water.
Get some powder gelatine and dissolve it in 50mls of boiling water, make sure it's completely dissolved before adding it to the cold water.
Set in fridge for around 2 hours and voila, if you want it to look like jelly add food colouring, I personally don't mind so don't bother.
Hope that helps (I stole that method from cherylxx on another thread so not to steal her thunder hehe)