Method #1⇝1] Download[↴] the Steam '.deb' archive to 'Downloads' directory
⇝2] Open Terminal & enter these commands - without the [#comments]
cd ~/Downloads [#changes the current terminal's dir. to '/home/USER/Downloads']
sudo dpkg -i steam_latest.deb [#to use sudo - user must be administrator]
sudo apt-get install python-apt [#tells 'apt-get' to install 'python-apt']
y [#if you are asked to continue choose yes (y)]
⇝3] Now, open the newly added "Steam" launcher to finish the installation
Method #2To be updated soon..