Enable Freeform window support in the app. Then turn Taskbar on with the toggle at the top of the app. You'll now see a floating app drawer icon in the bottom left of your screen.
Clear all your recent apps, because they need to be freshly launched to open in Freeform mode.
Go back to your homescreen and tap the Taskbar app drawer icon. You'll see the pop-up with the app list. Close it and reopen it a couple of times, until you notice the launcher disappear in the background and only the wallpaper remain.
You can now launch any app and it'll open in a freeform window.
I followed the steps to test it and at first, the apps were opening as usual, but I rebooted my Nexus 5X once more and it worked like expected. You can move the freeform windows around, tap and hold on any border to resize them, and switch between them as usual with the Recents key.
Taskbar is free so you can grab it now and test it out on your Android Nougat device. Freeform will work on phones and tablets, but obviously its benefits are limited on smaller screens.