I made a non-deduct contribution but filed the 8606 wrong. I filed the 8606 with my 2017 return, but the contribution was made 4/15/19 for 2018. How do I fix this?

1) submit a corrected 2017 Form 8606

2) submit a correct 2018 Form 8606 showing the basis that was on the 2017 form but belonged on the 2018 form.

Personally, I would use 2017 1040X and 2018 1040X to do these steps, respectively. In Part III you can explain what happened..

Next time, be more careful.