After you have drunk the sap of the willow tree and have been granted the wolf power, equip your hidden blades and go hunting for enemy "bluecoats". As your health degenerates when you are cloaked, it is best not to attack too large an enemy group; a patrol, or a convoy is ideal. sneak up from behind and activate the cloak just before reaching them. You should be able to take down at least two before they even notice something has happened. Kill a couple more in the confusion then run and hide until you health has returned, then go back and finish off any survivors.
Only the hidden blades allow you to stay cloaked while making a kill, any other weapon will expose you straight away. Also, bumping into anyone will also lose you your cloak.
25 kills will get you a nice shiny new silver trophy.