You'll report the nondeductible traditional IRA contribution made for 2017 on your 2017 Form 8606.
If you do the Roth conversion in 2017, the conversion will also go on the 2017 Form 8606. If you do the Roth conversion in 2018 instead, the basis in nondeductible traditional IRA contributions will carry forward to your 2018 Form 8606 where you'll report the Roth conversion.
