It does not matter if you have a single-page website or one that contains a million URLs, you need to audit each page for external http:// link references. This include anchor tags as well as any link tags you may have in your document's head.
By now, any reputable third-party service offers HTTPS support. If it does not, you may need to require them to offer HTTPS or find a new provider it they refuse.
As with the preceding Microsoft example , you should also make sure that all references to assets in your site are either HTTPS or are simply...