You can download and use the free SlingStudio Capture app to turn a compatible smartphone devices into a SlingStudio video source.
To see if your smartphone works with the SlingStudio Capture app, take a look at the article linked at the bottom of this page: What devices is SlingStudio compatible with?
Here's how to get the SlingStudio Capture app for your device:
The link below offers more information about device compatibility.