With the dropping of NSAPI in chromium and other browsers, the “easy” way to install GNOME shell extensions is gone (bug report). What’s left is to either get a deb package if someone created that, or install “manually”.
mkdir ./local/share/gnome-shell/extensions/
cd ./local/share/gnome-shell/extensions/
git clone https://github.com/romeshell/windows-alt-tab.git
Now hit Alt+F2, enter r and hit enter to restart GNOME shell and (re)load extensions.
Another useful tool is aptitude install gnome-tweak-tool.