You just need to add a CNAME record in DNS, whether it is in Cloudflare, GoDaddy or NameSilo:
Type | Name | Target | TTL |
CNAME | ▲ | ghs.googlehosted.com | auto |
▲ is the subdomain you want to use.
For example, my www has been resolved, which is www.v2know.com;
Now I want to parse blog.v2know.com to blogspot.
So I need to add a record in DNS panel like this:
Type | Name | Target | TTL |
CNAME | blog | ghs.googlehosted.com | auto |
Then customize the domain name in the settings of Blogger.