Military family who are FL permanent residents bought and registered a new car in CA in 2018. How can we deduct our registration fees since we aren't filing a FL return?

You can deduct the registration fees as an itemized deduction on your federal tax return. 

You can also deduct the sale tax your paid on the vehicle.

However, it is important to keep in mind that these are itemized deductions. If your total itemized deductions do not exceed the standard deduction ($24,000 for married couples, $18,000 for Head of Household filers), you will not notice a difference in your tax return.

To enter into TurboTax:

Federal 

Go to Deductions & Credits

Scroll to Sales tax to enter the sales tax you paid on the new car

Scroll to Cars and Other Things You Own to enter the registration fees