How to refine search using existing Site Collection columns

The Site Columns will be converted to Managed Properties automatically by SharePoint 2013. Depending on the data type the names of the columns will follow a standard naming convention detailed here: https://technet.microsoft.com/en-us/library/jj613136(v=office.15).aspx

You can assign properties to RefinableXXXXXXX fields (again based on data type) and make them available to the search center as refiners. https://technet.microsoft.com/en-us/library/jj679902(v=office.15).aspx

The final step is to configure the Refinement web part to use your properties. This is pretty simple and detailed here: https://technet.microsoft.com/en-us/library/jj679900(v=office.15).aspx#BKMK_ConfigureRPWP