This is pretty basic. Create a service that exposes the inputs you want to support. Use that input as input to the SP, if any. Have that service call the SP. Return the results in the format you want callers to use – probably JSON. Maybe XML. Maybe HTML.
Don’t get too wrapped up in the “API” term (which is something of a meaningless buzzword these days) – it’s just an HTTP call. Parameters can be specified in the URL. Or as an form in the body. Or as JSON or XML in the body. You could be flexible and allow the caller to specify the response type via the Accept header.
If you have more specific questions, please post them.
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