To set up a custom domain with Tumblr, please follow these steps:

1. Sign in to your Dynadot account.
2. Select "My Domains" from the left-side menu bar and click "Manage Domains" in the drop-down.
3. Check the box next to your domain name(s) and click the "Action" button.
4. Choose "DNS Settings" from the "Action" list.
5. On the DNS Settings page, select "Dynadot DNS" from the drop-down menu.
6. In the "Domain Record" field, select "A" from the drop-down menu and then enter IP address 66.6.44.4 in the "IP Address or Target Host" box.
7. In the "Subdomain Records (optional)" section, enter "www" as the subdomain, select "CNAME" from the drop-down menu and then enter domains.tumblr.com in the "IP Address or Target Host" box.
8. Press the "Save Settings" button to save your changes.