Here is a picture of the original top (forgot to take one of mine before cutting it!)
Firstly I cut the sleeves off by just following the stitching of where they were/
Next I drew a lower shaped neck-line with tailors chalk and cut round, making it a bit higher at the back.
Then I cut the armholes longer by just making a slit at the seam to just below the chest.
And there it is. By the way make sure you use good fabric scissors otherwise the edges will look horrible.