If your project is already under version control with SVN, then it is recommended that you use SVN externals to install Doctrine.
You can start by navigating to your checked out project in your terminal:
$ cd /var/www/my_project
Now that you are under your checked out project, you can execute the following command from your terminal and setup Doctrine as an SVN external:
$ svn propedit svn:externals lib/vendor
The above command will open your editor and you need to place the following text inside and save:
doctrine http://svn.doctrine-project.org/branches/1.2/lib
Now you can install Doctrine by doing an svn update:
$ svn update
It will download and install Doctrine at the following path: /var/www/my_project/lib/vendor/doctrine
Don't forget to commit your change to the SVN externals.
$ svn commit