Just bought a SSL certificate for one of my clients….
Here are some useful tutorial links.—

CSR Generation: Using OpenSSL (Apache w/mod_ssl, NGINX, OS X)
https://support.comodo.com/index.php?/Default/Knowledgebase/Article/View/1/66/

Nginx CSR Generation using OpenSSL
https://support.comodo.com/index.php?/Default/Knowledgebase/Article/View/801/0/nginx-csr-generation-using-openssl

Certificate Installation: Apache & mod_ssl
https://support.comodo.com/index.php?/Default/Knowledgebase/Article/View/637/66/

Certificate Installation : NGINX
https://support.comodo.com/index.php?/Default/Knowledgebase/Article/View/1091/0/certificate-installation–nginx