We'll get to the instructions, but first let's talk about what's actually here. Freeform Window Mode is just what we imagined. It's a dead ringer for Remix OS - multiple Android apps floating around inside windows - and might be the beginnings of a desktop operating system. It works on Android N phones and tablets, and once the mode is enabled, you'll see an extra button on thumbnails in the Recent Apps screen. To the right of the "X" button that pops up after a second or two, there will be a square shape - the same ugly placeholder art Google used for the split screen mode in the Android M Developer Preview.
Press the square symbol for an app and you'll be whisked away to a screen showing that app in a floating window that sits on top of your home screen wallpaper. The windows aren't floating above the Android desktop; it's just a blank wallpaper without any of your icons or widgets. The floating apps all have title bars like in Recent Apps. You can drag the apps around by the title bars or use the close and maximize buttons. Apps can be resized exactly how you would expect - press or hold on the edge of and all and move your finger, and you'll see the app change shape. Just like in split screen mode, apps will auto-switch between their tablet and phone layouts (with some apps dealing with this better than others). You can only resize in one direction at a time though; there doesn't seem to be a corner hotspot that will let you adjust the width and height.
It's honestly kind of amazing that we get to see both Apple and Google work on scaling up their mobile operating systems for desktop use, with the eventual end goal of replacing Chrome OS and OS X (get used to it, people - OS X is on its way out), and unify everything from desktop, to laptop, to tablet, to phone, in a single user interface that scales from top to bottom.
It's what Microsoft tried to do by scaling down, which honestly didn't pan out very well. We'll see if scaling up is a better approach, but exciting and interesting as it is to see this take shape before our very eyes, I still have my considerable doubts.