How to steal a story. How to file off the serial numbers and make it your own. . . . it's easier when you read a story and think "He
threw away that idea." or "Those minor characters are more interesting than the main plot" or "That development was a stupid waste when she could have done
this." or "That backstory would be a wonderful story on its own" -- but some of the same tricks may be needed as when you just want to steal the story.
There is nothing like practice. I am glad that my many, many, many pieces of juvenilia were handwritten -- in my virtually illegible hand-writing no less -- but it was good practice all the same.
The first trick is to
always change the names. Besides the psychological effect of making them your characters, besides the legal effect of escaping copyright, names have baggage. You may find that Catherine has dark hair, which will change the heroine's hair all on its own. (You can't count on the reader picking up your baggage, but you can find it useful if you write it in.)
It can be like prying a gemstone out of a rock. You may have to take several stabs at it -- put it down and come back a few days later, when you have more distance between you and the original story. It may feel like it can't be done, because the original story all hangs together. But it's a necessary step.
And then you start to look at what else you can change without changing what interested you in the first place.