Steps to take:
- Upgrade the Twitter Module from 6.x-2.6 to 6.x-3.0-beta2 or better
- Install OAuth 2.0 (OAuth 3.0 doesn't work with Twitter 6.x-3.0-beta2) - in the drupal Oauth settings be sure to set OAuth cryptography as "HMAC-SHA1" and not Plain Text or RSA.
- As ADMIN (User 1) Run update.php -> this will update the database to support the new version of the twitter module
- Register your application with twitter: In your Twitter application settings ensure you have "Application Type" set as Browser and the "Callback URL" should be http://yoursite.com/twitter/oauth
- Set up the Twitter Module on your website (at http://www.yoursite.com/admin/settings/twitter ) to use the registered Consumer Tokens: "Consumer Key" & "Consumer Secret"
- Login as the admin user whose twitter account has been setup
- Go to My accounts -> Edit -> Twitter Accounts (click on the "Make Global" link to the right for the global user you assign)