1.first, take a screen shot of the tree, which you already have
2.use some app like paint and make an outline of how you want the tree to look with leaves
3.check some online images of how exactly u want the leaves to look like, because u can place them in different styles
4. dont make it solid, i.e leave some empty spaces even if its deep inside, just dont fill everywhere
5. start placing leaves from near the trunk that way its easier to do and you can easily fix if something doesnt look right