(Source: Tumblr help center)
Your registrar should have instructions on how to implement the proper CNAME or A-record.
Okay, now head over to Tumblr:
- Click "Settings" under the account menu at the top of the dashboard, then choose the blog you’d like to update.
- Click the pencil to the right of the username section (Tumblr URL section for secondary blogs) and enable "Use a custom domain."
- Enter your domain (e.g., mywebsite.com) or subdomain (e.g., blog.mywebsite.com), then click “Test your domain.”
- Correct any problems if the test finds any, and click “Test your domain” until the test is successful.
- Hit “Save.”
After re-configuring your domain, you may need to wait up to 72 hours for the changes to take effect. If you visit the subdomain or domain and your Tumblr blog is showing up just fine, then congrats, all is well and good, stop fiddling around with this stuff.
If you see a Tumblr error page, but not your actual blog, it means you’ve at least correctly pointed the domain to Tumblr but haven’t configured your blog to use it yet. You should probably take care of that.
We’re unable to support many of the issues that crop up, so it’s best if you ask a friend who has done this before. Or consult a trusted authority figure. A firefighter, for example.