1. Create RSA Private Key and CSR pair.
    -> openssl req -new -nodes -keyout myserver.key -out server.csr

Note: Replace 'myserver.key' and 'server.csr' with names that are more descriptive to you.

2. Follow Prompts.
Note: If you need more information about the prompts then please see our KB Article on Apache CSR Generation thru OpenSSL

All in One Method

-> openssl req -new -nodes -keyout myserver.key -out server.csr -subj "/C=GB/ST=Yorks/L=York/O=MyCompany Ltd./OU=IT/CN=mysubdomain.mydomain.com"

Note: This is ONLY an example. Please modify it to suite your needs.

C  = ISO Country Code (2 letters)
ST = State/Province (Optional)
L  = City (optional)
O  = Your_Company_name
OU = Department (optional)
CN = Common Name (FQDN)

Related Articles
Certificate Installation: Java Based Web Servers (using OpenSSL)