If you don't have
openssl installed you need to install it first, using the following command:
sudo apt install openssl on Ubuntu
Or use the following command:
yum install openssl on CentOS/Redhat
sudo openssl req -x509 -newkey rsa:1024 -keyout ca.crt -out cert.crt -days 9999
You will then be invited to fill some details, as seen in the following screenshot, about the owner of the certificate, such as country of residence, state, company, city, email address, and the most important one is the Fully Qualified Domain Name...