We currently don't have a video or demo to walk through the process. The best method I can suggest right now is to make a custom activity that has a service reference to your SOAP service. You'll find the experience to be much more reliable with this method.

The reason workflow doesn't support SOAP is that its a lot more complicated to make the request from the activity because you have to build the more complicated SOAP message and there isn't documentation on what it should look like.

REST API services generally just need a URL to do something interesting or very simple post bodies, without the need for complicated headers.

Also, the REST API documentation tells you what you need to put into the activity in most cases.

If you have sample program that already uses the service you could try using a tool like Fiddler that will show you what the request should look like.