To create a self-signed certificate for courier-imap: 

openssl req -new -x509 -days 3650 -nodes -out imapd.pem -keyout imapd.pem

This will create a pem file with key and certificate in it. When asked for the common-name, enter the FQDN.

Don’t forget to specify the maildir path correctly in the imapd-ssl config file, as well as some other config parameters that are duplicated for imapd-ssl.

source.