You place the code in

app/code/local/Packagename/Modulename

Where Packagename/Modulename applied to your specific module (if you have the code it should already be in this structure)

Then, in

app/etc/modules 

Add an XML file named Packagename_Modulename.xml with the following contents

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <modules>
        <Packagename_Modulename>
            <active>true</active>
            <codePool>local</codePool>
        </Packagename_Modulename>>
    </modules>
</config>

Again, replacing Packagename_Modulename with the specific packagename and module name of the module you re installing.

You ll need to clear your Magento cache to see the changes take place.

The above won t really help you if what you have is a packaged magento extension, as this will already contain the correct directory structure magento requires.

If you have downloaded a packaged extension you will usually have the directory structure packaged in a tar.gz / zip file such as this:

app/

skin/

If you have a directory structure like this you can just paste the files into your Magento root directory, Magento will find the module for you.

you should ALWAYS perform a backup before installing an extension :-)

To install Magento 2 extension follow the instructions bellow:

1. Back up your web directory and store database.

2. Download installation package.

3. Upload content of the installation package to your store root folder via FTP/SFTP; install magento 2 extension.

4. Connect to your store root directory with SSH:

   cd path_to_the_store_root_folder
   

   Run 3 commands bellow:

   php -f bin/magento setup:upgrade
   php -f bin/magento setup:static-content:deploy
   php -f bin/magento setup:di:compile menu_console
   

5. Flush store cache, log out from the backend and log in again

   Execute the following command

   php -f bin/magento cache:clean menu_clear_cache
   

   Or go to Back-end > System > Cache Management. Click the following buttons to completely clear the store cache:

   • Flush Mangento Cache
   • Flush Cache Storage
   • Flush Catalog Images Cache
   • Flush Javascripts/CSS cache