Before starting it always is wise to read the documentation associated with an extension. Most extensions have homepages and forums, and it is a good idea to look at them first. If there is a README file included with the extension, you should read it.
For most extensions and most users, the procedure will be:
* Download the extension to your local machine * From the backend of your joomla site (adminsitration) select Extensions. * Browse for the package file * Click the install icon * Follow any instructions
There are some situations in which this procedure will not work.
Sometimes you need to unzip the file locally prior to installing. If you get an error saying that the file is not in the correct format, the need to unzip is a common cause of this. After unzipping try installing the individual items. Note that the files you upload using the installed still need to be zipped.
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.
For modules and mambots/plugins to work, you must make sure that they are pubished (1.0) or enabled (1.5).