MageDebugBar has only been tested with Magento 1.9, some earlier versions may work however Magento 2 does not, yet.

The composer install will create a symlink {magento root}/lib/vendor for composer based class loading (via vendor/autoload.php)

If you already have a file /lib/vendor in your Magento root then this installation will not be safe.

Note: you will need a github account as much of the code is retrieved from github and it may complain that you are requesting too much information and require your github username and password.

https://getcomposer.org/download/

Create the directory near to your Magento root. I use a directory structure like:

vhost/www      - Magento root
vhost/composer - Composer files

Create a composer.json file in the composer directory

{
    "prefer-stable": true,
    "require": {
        "bobd91/magedebugbar": "@dev"
    },
    "repositories": [
    {
        "type": "composer",
        "url": "http://packages.firegento.com"
    },
    {
        "type": "vcs",
        "url": "https://github.com/bobd91/magedebugbar"
    },
    {
        "type": "vcs",
        "url": "https://github.com/bobd91/php-debugbar"
    }
    ],
    "extra":{
        "magento-root-dir": "../www/"
    }
}

Make sure that the "magento-root-dir" points to your Magento installation.

Then run

composer install