After you have added a PPA to your system, the easiest way to install the packages from it is via browsing its contents using the Synaptic package manager. First make sure Synaptic is installed via:

sudo apt install synaptic

Then open the application from your applications menus, you should be able to browse package lists by origin like the following image, use it to install pacakges (right click on the package --> install):

If you added a PPA which contains only new versions of a package already installed on your system (E.g LibreOffice PPA containing newer version of LibreOffice, drivers or kernel PPAs...), then just running an upgrade would be sufficient to get the new version:

sudo apt update
sudo apt upgrade