>>2617271to learn how to draw there is the "reference material thread".
however they can be made even without being able to draw (the requests i made were made by taking a third image) something similar to pic related (ignore the fact that it's the same girl it's not necessary) just by using gimp (the nerfed version of photoshop)
you just need to have a (naked) breast in the same position and more or less similar skin (colour palette) which can then be adjusted, to create a variant.