Note: these instructions are for the Mac version of Thunderbird. Windows should be roughly equivalent.
Before you start, please have your e-mail address and password ready.
Step 1. Open Thunderbird, and choose Account Settings from the Tools menu. Click 'Add Account...' button in the bottom left corner. Choose 'E-mail account', then Continue.

Step 2. Enter your name and e-mail address. Click Continue.

Step 3. Check the IMAP option (recommended). For the domain of the e-mail server, enter 'mail.example.net', replacing example.net with your actual domain name. Click Continue.

Step 4. Enter your incoming username. IMPORTANT: enter your full e-mail address.

Step 5. Enter an Account Name. This can be any arbitrary name. Click Continue/Finish. You should be back to the Accounts setup page.

Step 6. You may or may not need to setup an SMTP server. If you already see one listed and marked 'Default', you're probably all set. This is the server used for your outbound e-mail. First try sending a message from one account to another that you control. See if the From line reads as expected (it should). If it does, your good to go. If not, no worries, highlight 'Outgoing Server' from the list on the left and click the Add button on the upper right.

Step 7. Add an arbitrary Description. The server name should be 'mail.example.net', again replacing the 'example.net' with your domain. Leave the 'name and password' checkbox un-checked. Click 'No' for 'Use secure connection'. Click OK. Test. You should be all set!
