1. To generate new private key and CSR use following command. Change domain_com with your value.
openssl req -new -newkey rsa:2048 -nodes -keyout domain_com.key -out domain_com.csr

2. While creating the CSR you will be prompted about following data. The values in [] are default one. If you click enter then the default values will be taken. All this values are required:
   • Country Name (2 letter code) [AU]
     State of company who register certificate. Use ISO two char notation, for example: DE
   • State or Province Name (full name) [Some-State]:
     State of company who register certificate.
   • Locality Name (eg, city) []:
     City of company who register certificate.
   • Organization Name (eg, company) [Internet Widgits Pty Ltd]:
     Company name with all additional leagal addings like Ltd / Sp. z o.o. / GmbH etc.
   • Organizational Unit Name (eg, section) []:
     If you are not sure the write just “IT Department”
   • Common Name (e.g. server FQDN or YOUR name) []:
     For wildcard domain *.example.com
     For single domain like www.example.com use www. prefix (not only example.com)
   • Email Address []:
     Email address. Not always required.
3. To check the generated CSR use:
   openssl req -in domain_com.csr -noout -text
Mind the signature. It must be sha256: Signature Algorithm: sha256WithRSAEncryption

If you want to createMultiple Domain certificate request then read this article: https://ethitter.com/2016/05/generating-a-csr-with-san-at-the-command-line/