Details

What is it?

Basically, try and write a new ending to a story that you like, to make it better for you. For example: The author kills the protagonist, and you wished something else had happened.
It's pretty fun.

What sort of space will you need?

This is a silly section. It requires a little bit of common sense. If a 'hack' (god I hate that term, and the term 'hacker') says write something, you aren't going to need a Donald Trump's ego size space.
Write it wherever you see fit, in the park or at a desk.

When could it take place?

Fair enough, this section could be useful. Do it when you have actually read the book. I know, great advice.

How?

Things you will need:
Pen/Pencil/Computer
Paper if you are using a pen or pencil
Something hard to write oN
A brain

Step 1
Get you writing utensils together

Step 2
Let your imagination flow