First you will need to install composer, because composer is used to do the install of Drush now.
# curl -sS https://getcomposer.org/installer | php # sudo mv composer.phar /usr/local/bin/composer
To install Drush 6.x (stable):
# composer global require drush/drush:6.*
To install Drush 7.x (dev) which is required for Drupal 8:
# composer global require drush/drush:dev-master
Now you have drush installed, just need to add the directory to your path
# sed -i '1i export PATH="$HOME/.composer/vendor/bin:$PATH"' $HOME/.bashrc # source $HOME/.bashrc
Drush executable is installed in the bin path of composer, which is under the users home directory.
That is it
Now you can just run drush, you probably want to change to the root directory of a drupal install and run it
# drush status
or to learn more type
# drush topic