OK, here we gooo (:
Right, take the clothing you're going to transform..
cut it to the length you want..
then cut the front to the desired shape.
The further down it is towards the green lines the more off the shoulder it will be.. (just thought I'd say that in case you wanted more of a shawly thing..)
and the further in it is towards the blue lines the the closer the front parts will be.. although obviously depending on how the clothing fitted you before..
Yeah, so basically.. cut it out d:
then just add two bits of material onto the front edge.. and ta da!