First You need to acquire a certificate matching the URL of your website.
I prefer using https://www.startssl.com/ because validation process is fast and the certificates I use are free.
Next you login to your Synology and go to Contol Panel -> Advanced Mode -> Security -> Certificate.
Here You can import your cerficate or you can create a certificate signing request (CSR) for ordering new certificates.
Once imported, Click “Configure” and select the new certificate for your website
Now go to Main Menu and choose “Web Station” open the Virtual Host for your website and select “HTTPS” and use default port 443
Last step, go back to Control Panel and choose -> Network -> DSM Settings
Select “Automatically redirect HTTP connections to HTTPS (Web station and Photo….