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.