"so do you enter $6,000 in both places"

Both places? Once you enter the data, confirm it looks right by examining Form 8606, as well as the 1040. A Backdoor Roth is implied to be a nondeductible contribution to a Trad IRA immediately converted to Roth. The reason it would not be taxable is because the nondeducted contribution is basis (already taxed) and immediately moving it as conversion would avoid having any earnings on that amount. The reason it would be taxable, is that there is other non-Basis money in the similar accounts (as you see on the Form 8606, where you enter FMV) and that means any conversion (backdoor or not) will be prorated for tax purposes.