Dynamic sorting in Stored Procedure using MySQL

Hello I would like to ask how to use Dynamic Order By in Stored Proc using MySQL.

http://www.aspsnippets.com/Articles/Custom-Paging-in-ASPNet-GridView-using-MySql-Stored-Procedure.aspx

DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `GetCustomers_Pager`(
   _PageIndex INT
   ,_PageSize INT
   ,_SortExpression VARCHAR(50)
   ,_SortDirection VARCHAR(50)
   ,OUT _RecordCount INT
)
BEGIN
       SET @RowNumber:=0;
 
       CREATE TEMPORARY TABLE Results
       SELECT @RowNumber:=@RowNumber+1 RowNumber
              ,CustomerID
              ,ContactName
              ,CompanyName
       FROM Customers;
 
       SET _RecordCount =(SELECT COUNT(*) FROM Results);
 
       SELECT * FROM Results
       WHERE RowNumber BETWEEN(_PageIndex -1) * _PageSize + 1 AND(((_PageIndex -1) * _PageSize + 1) + _PageSize) - 1;
 
       DROP TEMPORARY TABLE Results;
END$$
DELIMITER ;

I've been Stock on how to apply the SORTEXPRESSION and SORTDIRECTION.

Thanks!