For prior year Form 8606, should I simply mail the form to IRS and wait for their response? Or should I enclose a letter ? Will I be billed later for the penalties?

I was unaware of the requirement to file Form 8606 for non-deductible IRA contributions. I made these on 3 occasions in the past. I used TurboTax (Web) on these years and may have missed the question. I checked the returns and this form has not been filed. I downloaded the forms for the past years and filled the details in.