Usually we can add a new PPA repository by adding it into Software Sources (software-properties-gtk) either from Ubuntu Software Center or Synaptic. But actually we can add same PPA manually. This manual method will help user to easily remove unused PPA later.
Search for Launchpad PPA Page
First, search for a PPA. For example, we can type shutter ppa on Google if we want Shutter application PPA. We must find an URL like this https://launchpad.net/~shutter/+archive/ubuntu/ppa and we must see the page contains "Technical details about this PPA" section. Otherwise do search again.
Select Ubuntu Version
At Technical details section, select Ubuntu version. For example, we use 14.04 so we select Trusty 14.04. This selection is helpful so we just need to copy the text.
Copy PPA Source List
After selecting, we will find text lines like this. These are PPA URLs or Ubuntu calls them sources list. Notice that deb line is for binary package, and deb-src line is for source code package. We install binary package so copy the deb line only. Copy only the first line.
deb http://ppa.launchpad.net/shutter/ppa/ubuntu trusty main deb-src http://ppa.launchpad.net/shutter/ppa/ubuntu trusty main
Paste PPA into Your sources.list File
Perform command sudo gedit /etc/apt/sources.list on the Terminal. Gedit text editor will appear to open our Ubuntu sources.list. Paste text into the new line. Our sources.list fill content will be like this. Notice the last line.
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted multiverse universe deb http://security.ubuntu.com/ubuntu/ trusty-security main restricted deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted deb http://ppa.launchpad.net/shutter/ppa/ubuntu trusty main
Reload
After adding a new repository (in this case, PPA), do command sudo apt-get update. This will reload the new repository so apt-get later can install from it.
Install
If the PPA is Shutter, then we will install shutter package. The command is sudo apt-get install shutter. Otherwise, instead using Terminal we can install it with Ubuntu Software Center or Synaptic.