This is very easy, first make sure your website is working with your server. Point the A or AAAA records to your server for your domain, then you can go to your website in Ploi and select "SSL" on the left hand side.
Now press the big "LetsEncrypt" button, you will be prompted for which domains you want to request, usually this is pre-filled so you can just click "Save" to get the certificate from LetsEncrypt.
If this process fails, we will detect this and send you a e-mail about it containing the error. 95% of the cases that a certificate is failed to be requested is due to the fact that the DNS record is not been setup correctly to your server.