start with an image you would like to photoshop
green cat
then you use lasso magnet tool from the other picture and cut the bulb out then paste on the other picture


realistic bulbasaur
you paste the bulb and transform to fit it just right.
bulbasaur cat
once you have the bulb placed you can asdjust the color to make it look right


once everything is dont and corrected it should look finished and pretty