Step 1: The .PEM file setup

You will receive your certificate file(s) by e-mail. Your certificate file has to be a .pem file. If you also received an Intermediate Certificate then you have to concatenate this with the Domain Certificate and your Private Key file into one single .pem file. Make sure all the information is included, without any spaces or blanks, see  below. 

-----BEGIN CERTIFICATE-----
 (your_domain_name.crt)
 -----END CERTIFICATE-----
 -----BEGIN CERTIFICATE-----
 (your_intermediate.crt)
 -----END CERTIFICATE KEY-----
-----BEGIN RSA PRIVATE KEY-----
 (your_domain_name.key)
 -----END RSA PRIVATE KEY-----

Step 2: Configuration file

Open the Cyrus IMAP configuration file, imapd.conf (usually located in /etc/imapd.conf) and modify or add the following lines:

tls_cert_file: /path-to-file/server.pem
tls_key_file: /path-to-file/server.pem
tls_ca_file: /path-to-file/server.pem
tls_ca_path: /path-to-ca-map/

Make sure the files are readable.
For example: chown cyrus /var/imap/server.pem

Step 3: Restart Cyrus