You should complete your 2018 income tax return before you complete your 2019 that way your information from the prior year can be transferred to this year's return. 

If you started your 2019 return and filed it first, then just complete your 2018 return and file it also- but on the 2019 return, it will ask if you have to complete any prior year returns, as long as you answered yes to that question you will be fine.