For a domain of the form docs.notionlayer.com, please follow the guide for your domain provider:
Generic/All other providers (Based on Cloudflare)
Go to DNS Records configuration of your domain provider.
Fill in the form as follows:
Type:CNAME
Name: Sometimes called “Path” should be set to docs , ie. the first part of the domain.
Target: This should be set to fork.notionlayer.com.
Note that since this provider (Cloudflare) automatically adds the . to the end of fork.notionlayer.com as seen below you can omit it in the target settings.