Deseasonality question

I have a set of values over 12 months.
I need to deseasonalize the values.
I did the average of all twelve months (z)
Then took the values of each month and divide it by "z".
By doing so, it gives me my seasonal index.
How do I get my value deseasonalized?
If i divide the average of each month by my seasonal index, it will simply give me my z back again?