I made an after tax contribution to my 2018 HSA from my checking account in 2019. Do I report that on my 2018 taxes or my 2019 taxes?

You can contribute to your HSA for tax year 2018 and claim it on your 2018 tax return if the contribution is made before April 15, 2019. Make sure to indicate to your HSA administrator that the contribution is for tax year 2018.