SSIS ,Pass Database name SQL Query from Variable

Hi Experts ,

I need to change my Database name in SSIS dynamically .
I have seen so many article Online ,But did not get complete Reply step by step.
I am working on SSIS 2012 .

My Requirement : I need to use SQL Query to Fetch Data in OLEDB SOURCE ,
But Database name will change when I move to Other Environment
I have a Query

SELECT    One.Col1,Two.col2
from        DB1.dbo.tbl1 One
Left Outer Join DB2.dbo.tbl2 Two on Some Condition.

I have create 2 Variable in SSIS Package Level i.e
VAR_DB1 ,Type:String ,VAlue :DB1
VAR_DB2,Type: String ,VALUE:DB2

What I need to Do Further?,Which access Mode to Use ? What change I need to make in my above SQL Query to accept Variable value etc etc  ?
Plz Let me know all the step by Step approach if possible as I need to Load above data in Target table