Step 1 - Install lets encrypt

Execute the following commands to install let's encrypt on ubuntu:

sudo apt install certbot python3-certbot-nginx

Step 2 - Allow HTTPS through the firewall and configure Nginx virtual hosts

If you are using Nginx, enter the following commands to allow Nginx over the firewall:

sudo ufw allow 'Nginx Full'

Step 3 - Obtain an SSL certificate

Run the following command to obtain an SSL certificate

sudo certbot --nginx

This command shows you some prompts and finally installs an SSL certificate on the domain of your choice.