I'm writing a workflow in SPD2013 in which one of the events that should happen is kicking off another workflow that is cross-site. I know that one way to do so using SPD2013 is using the Web HTTP services, but I looked online and am not sure how to use it.
HTTP Web Service allows you to access anything in the SharePoint site using APIs. The main functionality of what you want to do may differ, but essentially you will get some details from the Workflow column in one list and create a condition which will start the new workflow.
An example would be:
You would be able to use the request to create conditions based on the values you obtain.