The documentation for writing something yourself is at https://github.com/ietf-wg-acme/acme/blob/master/draft-ietf-acme-acme.md#dns
I’d suggest using one of the alternative clients some of which include DNS verification. I know that the two bash scripts work for DNS verification, so for automation on the same server I’d suggest LetsEncrypt.sh or for automation on a remote server getssl.