First, you should login to the PEAR website using the account you've been given (see the first step).
If this is the first release of your package, you have to first register it. Registration is done through the form at http://pear.php.net/package-new.php. If you are in doubt about the category to choose, consult the community (preferably through the pear-dev mailing list).
Now, finally, you can release your package. This is done again using the convenient web interface. Use the form at http://pear.php.net/release-upload.php.
That's it! Your package has been released, an announcement has been sent to the mailing lists and everyone is happy :).