Once you install it, simply hit the PiP logo in the upper right corner of the browser when the video is playing.
This will shrink it down to the corner, letting you continue watching videos even from other tabs.
Shrunken video will have limited controls
It is also important to note that the shrunken video will have limited controls when you use the PiP mode.
Basically, it will offer you just two options: either go back to the full-screen player or pause the clip. There will be no button to forward or replay the video or to adjust the volume level of the player.