The best way to install and configure a Symfony2 project is described in the official documentation as follows:

Mac OS X / Linux

$ sudo curl -LsS http://symfony.com/installer -o /usr/local/bin/symfony
$ sudo chmod a+x /usr/local/bin/symfony 

Windows

c:\> php -r "file_put_contents('symfony', file_get_contents('https://symfony.com/installer'));" 

Then you could use the Symfony binary to build the right scaffolding:

$ symfony new my_project 

Given you've already installed composer up and running and it's accessible globally, you can simply create new Symfony projects as stated in the official documentation.

Now you can create a new Symfony project with composer:

composer create-project symfony/framework-standard-edition my_project_name

This will create the project in the current directory you're in.

If you want to create the project with a specific version of Symfony, you can add a parameter with the version number:

composer create-project symfony/framework-standard-edition my_project_name "2.8.*"

Hint: If you're thinking that composer won't do anything, add the -vvv flag to the command. That way, you'll receive detailed information about what composer is doing right now.