Variable to store Flat File Path

Please let me know the steps to store Flat File Destination Path in a variable .

The file and its metadata is configured at design time.

I have a package which has 50 Data Flow Tasks .Each of the data flow tasks has ODBC source ,transformation and Flat File destination.

The path where all the 50 Flat Files has to be saved it same but the file name and metadata is different.

So I want to set only the path as variable so that I can change it whenever required.

Please help me with steps to 

i) assign a variable to flat file path

ii) Use it in each of the flat file destination I create in each data flow task