Where do I write off bad debt on rental property? do I attach it to the property? Do I have to have a judgment or just a summary of the rent owed?

You don't deduct (write off) something you never received, you just have less income to report.

What is a bad debt if not something you haven't received?

Lets say if you had received rent all year, you would have received 15,000.  But your renters are flakes and didn't pay you for 3 months.  So you wouldn't write anything off, you just report rental income of 11,250 rather than 15,000.

You "could" report 15,000 as income, then deduct the 3750 you never received, but the end result is the same.

 Generally, to deduct a bad debt, you must have previously included the amount in your income, or loaned out your cash.

https://www.irs.gov/taxtopics/tc453.html