Maximus Gnome Shell Extension + Gnome 3.18

Try "official" extension page. Maybe it supports gnome 3.18 already: https://extensions.gnome.org/extension/844/maximus-two/
If official page still not supports - then go to https://github.com/diaevd/GnomeExtensionMaximusTwo
and click "Download ZIP" button
or use this direct link: https://github.com/diaevd/GnomeExtensionMaximusTwo/archive/3.18.zip

Also, you need to install "gnome-tweeak-tool" if you not installed this useful tool before

sudo dnf install gnome-tweeak-tool

Open gnome-tweak-tool application (for example, press Alt+F2 and type "gnome-tweak-tool"(without quotes)
Go to "Extensions" tab
Click on the "folder" icon in "Install shell extension" section (at the bottom of the list)
Select file "GnomeExtensionMaximusTwo-3.18.zip" downloaded on previous steps
Close "gnome-tweak-tool" application
Press Alt+F2 and type "r" (without quotes", press Enter
Open gnome-tweak-tool application again and go to "Extensions" tab
Enable "Maximus Two" extension using slider.

fedora 23 + nvidia proprietary drivers

Currently (7 november 2015) if you update fedora 22 to fedora 23 and if you have nvidia proprietary drivers - you will face crashing gnome 3.18
This happens because nvidia drivers are not working with latest xorg.
So you have 2 solutions:
1) wait till nvidia will release updated driver
2) downgrade xorg after fedora 22 -> fedora 23 upgrade

Let's imagine you selected "2". Then you need to follow these instructions:

dnf remove xorg-x11-drv-nouveau
dnf downgrade xorg-x11-server\* --releasever=22 --allowerasing
echo "exclude=xorg-x11*" >> /etc/dnf/dnf.conf
dnf install akmod-nvidia --releasever 22
systemctl enable akmods
reboot

I "gathered" these instructions from following places:
https://lists.rpmfusion.org/pipermail/rpmfusion-users/2015-November/000705.html
http://rpmfusion.org/Howto/nVidia