I cannot find information on adding https:// security to my website using SiteSpinner Plus. Any useful advise welcome.
SS does not control how the server handles http or https requests. This needs to be set up on your server.
If you want to use the https protocol on your site, you need to purchase and install an SSL certificate. You should be able to get a certificate from your host, and they should install it for you. If your host does not provide this service, then you can get a certificate from other sources, such as GoDaddy, DigiCert and InstantSSL. You will also be responsible for getting your certificate installed on your site.
After installing the certificate, you will need to modify the .htaccess file on your site to redirect normal http requests to https requests, or provide some other sort of redirection.
There are various levels of SSL, which has more to do with branding than with the actual security provided. Costs vary based on the level of "security" you purchase.
I would discourage you from using a self-signed certificate. While such a certificate will provide the security you need, since it is self-signed it will not be trusted by browsers and your visitors will need to manually allow access to your site.