A TSP is pre-tax, is not a Roth. If you rolled over your TSP account into a privately owned IRA, that is all pre-tax at this point.
For 2022, if you contributed $1000 to the IRA, you either took a tax deduction or you didn't. If you did not take a tax deduction, then you should have a form 8606 with your 2022 return that documents that you now have a $1000 after-tax basis in the IRA. That form 8606 is what you use to calculate the pro-rata tax on the conversion, and you will also get a new form 8606 when you file your 2025 return (because any conversion now will be in 2025) that will document the calculation, and show the remaining amount of the after-tax basis (if you don't convert the entire IRA all at once).
If you think you did not take a tax deduction for the $1000 contribution but you don't have a form 8606 with your 2022 tax return, you need to file an amended return that either takes the tax deduction or reports the non-deductible contribution on form 8606.
