I have gambling losses for 2018 as well as 2019 that were never reported. How do I report them this year?

You can only use 2018 gambling losses on a 2018 tax return.  Did you itemize on that return?  if not, the losses will have no effect.   If you were able to itemize for 2018 then you can amend your 2018 return.

Gambling

To enter the W-2G  or other documents For your Gambling winnings--Go to Federal>Wages & Income>Less Common Income>Gambling Winnings

You can enter your winnings, and then keep clicking through the interview to enter gambling losses.

https://www.irs.gov/help/ita/how-do-i-claim-my-gambling-winnings-and-or-losses

Gambling winnings are taxable income.  Losses are an itemized deduction.  If you do not have enough itemized deductions to exceed your standard deduction, your losses will have no effect.

https://blog.turbotax.intuit.com/income-and-investments/how-are-gambling-winnings-taxed-8891/

https://ttlc.intuit.com/questions/1900352-can-i-deduct-my-gambling-losses

2019 Standard Deduction Amounts

Single $12,200   (+ $1650 65 or older)

Married Filing Separate  $12,200   (+ $1300 if 65 or older)

Married Filing Jointly $24,400   (+ $1300 for each spouse 65 or older)

Head of Household $18,350  (+ $1650 for 65 or older)