OK, from kde to Cinnamon here is how you do it.
General instructions on how to install an additional desktop can be found here.
Here are the specifics for your situation:
sudo pacman -S cinnamon cinnamon-wallpapers cinnamon-sounds gnome-terminal manjaro-cinnamon-settings adapta-maia-theme kvantum-manjaro vibrancy-icons-teal
You will get a message about replacing
manjaro-cinnamon-settings, press 'Y'
Create a new user replacing "bob" with the username:
sudo useradd -mG wheel bob
Please note this puts the new user in the wheel group which essentially provides admin rights as your original user would have had.
Reboot. At the login screen, select the new user and select Cinnamon. Then login.
Going forward, use your original user for kde and the new user you create for cinnamon.
You may lose some theming in KDE but you can just go into settings and apply it.