Answering here in case others are searching for a solution:

ServerPilot uses Nginx as the public facing web server and proxies the requests to Apache. So, we have to add our SSL configuration to Nginx.

Steps:

Login to the server using SSH

Create a directory to hold the certificate and key files.

cd /home
mkdir -p certs/domain_name

Copy the certificate (.crt) and private (.key) files to this directory. Replace domain_name with your domain name.

Add custom SSL configuration here:

cd /etc/nginx-sp/vhosts.d
nano APP_NAME.ssl.conf

Replace APPNAME with your actual app name (website). Put this inside the file APPNAME.ssl.conf:

###############################################################################
# Install SSL Certificate
###############################################################################

server {
    listen       443 ssl;
    listen       [::]:443 ssl;
    server_name
        www.DOMAIN.com
        DOMAIN.com
      ;

    ssl_certificate /home/certs/domain_name/certificate_file.crt;
    ssl_certificate_key /home/certs/domain_name/privatekey_file.key;

    root   /srv/users/serverpilot/apps/APP_NAME/public;

    access_log  /srv/users/serverpilot/log/APP_NAME/APP_NAME_nginx.access.log  main;
    error_log  /srv/users/serverpilot/log/APP_NAME/APP_NAME_nginx.error.log;

    proxy_set_header    Host              $host;
    proxy_set_header    X-Real-IP         $remote_addr;
    proxy_set_header    X-Forwarded-For   $proxy_add_x_forwarded_for;

    include /etc/nginx-sp/vhosts.d/APP_NAME.d/*.nonssl_conf;
    include /etc/nginx-sp/vhosts.d/APP_NAME.d/*.conf;
}

As usual, replace APPNAME, domainname, certificatefile and privatekeyfile with your own values.

Restart Nginx

service nginx-sp restart

That's it. The SSL certificate is installed.

NikhilSharma thanks heaps!!

Here's the final .ssl.conf file with the changes incorporated from the comments above.

server {
listen 443 ssl;
listen [::]:443 ssl;
server_name
www.DOMAIN.com
DOMAIN.com
;

ssl on;

ssl_certificate /home/certs/domain_name/certificate_file.crt;
ssl_certificate_key /home/certs/domain_name/privatekey_file.key;

root   /srv/users/serverpilot/apps/APP_NAME/public;

access_log  /srv/users/serverpilot/log/APP_NAME/APP_NAME_nginx.access.log  main;
error_log  /srv/users/serverpilot/log/APP_NAME/APP_NAME_nginx.error.log;

proxy_set_header    Host              $host;
proxy_set_header    X-Real-IP         $remote_addr;
proxy_set_header    X-Forwarded-For   $proxy_add_x_forwarded_for;
proxy_set_header    X-Forwarded-SSL on;
proxy_set_header    X-Forwarded-Proto $scheme;

include /etc/nginx-sp/vhosts.d/APP_NAME.d/*.nonssl_conf;
include /etc/nginx-sp/vhosts.d/APP_NAME.d/*.conf;
}

Make sure you replace APPNAME, domainname, certificatefile and privatekeyfile with your own values.

If your paths are incorrect you will either get an error when your restart Nginx or a 403 error when you visit your website.

Restart Nginx

service nginx-sp restart

I recently wrote a detailed tutorial for it.. The actual SSL installation and configuration on ServerPilot will take less than 10 minutes. You can check it out over here:

https://www.blogmehow.com/how-to-manually-install-ssl-on-serverpilot-free-plan-1331/

Let me know if it helps...

Awesome dude, thank you very much for taking the time to help little noobs like me, lifesaver! hahaha @NikhilSharma

Thanks a lot!
Do you also know how to manage CA certs?

e.g. with comodo I have these CA certs:
COMODORSADomainValidationSecureServerCA.crt
COMODORSAAddTrustCA.crt
AddTrustExternalCARoot.crt

@NikhilSharma hi again! I have a little question more, if you have time =)...

In the case is a Wildcard SSL is the same steps above but for each subdomain?

Thanks again.

Should I replace :
www.DOMAIN.com
DOMAIN.com
as well ?

Thanks

In case it helps anyone, I had to add the following at the end of the proxy section to get it to work:

proxy_set_header    X-Forwarded-SSL on;
proxy_set_header    X-Forwarded-Proto $scheme;

Hi all

From the moment I have this APP(replaced).ssl.conf file, I get an ERRCONNECTIONREFUSED error. This is in app.conf:

server {
    listen       80;
    listen       [::]:80;
    server_name
        xxxxx.be
        www.xxxxx.be
      ;

    root   /srv/users/serverpilot/apps/sla/public;

    access_log  /srv/users/serverpilot/log/sla/sla_nginx.access.log  main;
    error_log  /srv/users/serverpilot/log/sla/sla_nginx.error.log;

    proxy_set_header    Host              $host;
    proxy_set_header    X-Real-IP         $remote_addr;
    proxy_set_header    X-Forwarded-For   $proxy_add_x_forwarded_for;

    include /etc/nginx-sp/vhosts.d/sla.d/*.nonssl_conf;
    include /etc/nginx-sp/vhosts.d/sla.d/*.conf;
}

This is in app.ssl.conf:

server {
listen 443 ssl;
listen [::]:443 ssl;
server_name
www.xxxxx.be
xxxxx.be
;

ssl on;

ssl_certificate /root/certs/xxxxx.be/ssl-bundle.crt;
ssl_certificate_key /root/certs/xxxxx.be/ssl.key;

root   /srv/users/serverpilot/apps/sla/public;

access_log  /srv/users/serverpilot/log/sla/sla_nginx.access.log  main;
error_log  /srv/users/serverpilot/log/sla/sla_nginx.error.log;

proxy_set_header    Host              $host;
proxy_set_header    X-Real-IP         $remote_addr;
proxy_set_header    X-Forwarded-For   $proxy_add_x_forwarded_for;
proxy_set_header    X-Forwarded-SSL on;
proxy_set_header    X-Forwarded-Proto $scheme;

include /etc/nginx-sp/vhosts.d/sla.d/*.nonssl_conf;
include /etc/nginx-sp/vhosts.d/sla.d/*.conf;
}

Any ideas?

I wanted to chime in on this topic, because this really saved my ass today!
I had an SSL Wildcard installed (that expired) and needed to install a new SSL, but was installing separate SSL's on the FQDN and a subdomain and each would get there own SSL.
So, everything, information wise was already in place in the .conf files, except I used the built-in ServerPilot method last time and the difference in what I did to get it to work was just where the SSL.crt and SSL.key were located.

It was trying to look for the SSL crt/key in etc/ssl...
I used the following and it worked like a charm:

ssl_certificate_key      /../srv/users/APP_NAME/certs/DOMAIN_NAME/ssl.key;
ssl_certificate          /../srv/users/APP_NAME/certs/DOMAIN_NAME/ssl.crt;

Thanks for the amazeballs tutorial, everything works again and I don't have to deal with the awful ServerPilot support team, which decided it's probably fine to not respond to tickets on the weekend.... what could go wrong?

:D

Got one site up with SSL on my droplet after following these directions. Thanks so much! Lifesaver!! However, does anyone know to host a 2nd SSL cert on the same droplet (IP) for a second website? I found this on how to add multiple SSL certs, but I am not sure how much has already been done from the tutorial above plus it's over 4 years old, so not sure if anything has changed or there is an easier way. I am quite a noob to this. Thanks for your help in advance.

I have this question!