Firstly select your single leaf object that consists of all the trees leaves.

Go into edit mode and press "A" to select all the faces of all the leaves.

Press "P" which brings up the Separate selection box.

Choose "By Loose Parts".  This will separate all the leaves from each other.

When doing this with an object like the leaves on a tree be prepared to wait awhile for your computer to process this command as it can take some time due to the number of objects that need separating.  Also I suggest that when you select your leaf object right at the start you move this object to its own layer as you will have hundreds of individual leaf objects at the end of this process.  This will help when you follow the steps in the video you linked too.  

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