First you should set CloudFlare's name servers at your domain registrar.
This is you CloudFlare's Nameservers
This is your example of registrar's Nameservers
After that you should set A name 18.104.22.168 at CloudFlare , if you use main domain as a blog. Or CNAME domains.postach.io, for subdomain.
I've set CNAME only, 'cause I use subdomain:
More information about Postach.io custom domains is here.