From the backend of your joomla site (adminsitration) select Installers and then the type of extension (module, component, mambot/plugin, site template, administrative template, language

-Browse for the package file

-Click the install icon

-Follow any instructions

Sometimes you cannot use the automated installer. For example, very large extensions may exceed the maximum upload size allowed by your host.

In this case, unzip all of the files locally. Then transfer the files to a folder in the the install directory(for example administrator/components/com_installer/components) for the type of extension you are installing (using FTP). Then use the installer, but select "install from directory" indicating the correct folder name.