Note: you can't install DavMail directly on an iPhone, you need a separate DavMail server, see iPhone setup.
Open iPhone Settings application and choose Mail, Contacts, Calendar:
Then select Add Account...:
DavMail is not an Exchange ActiveSync server, press Other:
Choose Add CalDAV Account:
Enter your DavMail server hostname in the Server field, append ':' and port if DavMail HTTP (Caldav and Carddav) port is not 80, provide your username and password then press Next:
On the account page, open Advanced Settings:
Enable SSL and check port, make sure Account URL is https://davmailhost:firstname.lastname@example.org: