This is a straight forward tutorial, so the setting up will also be straight forward.
Create a New ActionScript 3.0 Flash Project. The stage size and color don’t matter, just use what you are confortable with.
I use FlashDevelop for coding, but also this could be done in any AS editor, like Flash Pro (or any text editor, maybe Notepad 😉 ). So, create a Class file, make sure your code looks pretty much like mine; see below. I called mine “Movement”. (If you’re using Flash Pro, check out this guide to creating a class.)