While you have the option to "let TurboTax do it for you", I don't recommend it. That's because there are some things that can't be prorated correctly or at all by the program.
For your typical rental expenses such as repairs, maint, advertising, etc, you only enter costs incurred after converting it to a rental. Now while the program will prorate the property taxes and mortgage insurance, it seems to have difficulty with the property insurance. That's because property insurance is not a SCH A itemized deduction and never has been. So you pro-rate the property insurance for the rental time only.
