Comodo Cert import

Importing certificates for encrypted/signed ssl email.  Sorry if I missed it but i haven’t seen the answer while searching the FAQs.  Is there a procedure or cheat sheet I can follow to install a Comodo free email cert? It ends in ‘crt’ and the program doesn’t seem to recognize or install it.  To be honest, I’m a noon teaching myself encryption technology and I’m stuck  All help appreciated.