Follow the steps below for pointing the domain to tumblr
1. You need a valid domain name.
2. At least a hosting package or DNS management tools to point domain to tumblr.
3. Fill in below DNS record to DNS zone.
4. Once done the setting, kindly access to your tumblr dashboard to perform below steps.
a. Click Settings (the gear icon) at the top of your Dashboard.
b. Click the blog you’d like to update on the right side of the page.
c. Click the pencil to the right of the username section and enable "Use a custom domain."
d. Enter your domain (e.g. mywebsite.com) or subdomain (e.g. blog.mywebsite.com).
e. Click “Test your domain.”
f. Correct problems if the test finds any, and click “Test your domain” until the test is successful.
g. Hit “Save.”
5. After re-configuring your domain, you must wait up to 72 hours for the changes to take effect. When you visit the subdomain or domain, you should see a Tumblr error page — this means that the domain is correctly pointing to Tumblr, but that your blog hasn’t been configured to use it yet.