Below are the steps I have followed to configure SharePoint 2013 workflow:
1. Understand workflows in SharePoint 2013: http://technet.microsoft.com/en-us/library/jj219638.aspx .
2. Initially, the Workflow Service in SharePoint 2013 is not connected for it requires a compatible workflow service such as Workflow Manager, and the SharePoint Designer 2013 has only SharePoint 2010 Workflow platform option.
3. Download and install Workflow Manager 1.0. The download link and other information are available at http://www.microsoft.com/en-my/download/details.aspx?id=35375 .
4. Download and install Workflow Client in SharePoint server. The download link and other information are available at http://www.microsoft.com/en-my/download/details.aspx?id=35375
5. Configure workflow in SharePoint server. Please refer to http://technet.microsoft.com/en-us/library/jj658588(v=office.15) for the guide.
6. At this point, the Workflow Service will already be connected, and SharePoint 2013 Workflow platform option in SharePoint Designer 2013 will already be available.