Let's say I wanted to host a Carrd design on home.senrigan.io (so aside from www.senrigan.io).
If you follow the instructions exactly, you end up with a CNAME that doesn't work and your subdomain won't work! The important caveat was to have patience and to also take out the CNAME record.
1. Only setup the A record on CloudFlare. I recommend using DNS only and setting the TTL to 2 minutes (until you know it works for a few days). Then you can turn CloudFlare's proxy back on if you wish.
2. You should NOT setup the CNAME record if this is a subdomain.
3. On Carrd - Hit Publish. Wait an hour if it doesn't immediately work. In my first attempt, I didn't realize that the changes were a bit slow to propagate.
4. You may see some https:// errors, but give it a few hours for LetsEncrypt to figure itself out. The key is patience!