Pretty straight forward.
Pick Time Trial from the Solo menu. Choose either F1 2019, Classic or F2 2018 cars.
After you've picked your car, choose any track, but before accepting, make sure you pick the Wet tab at the top. After you've loaded in, set a time in the wet conditions and the trophy/achievement will pop.
Note: I am not sure if 'dirty lap' counts, but if you set a clean lap, it will definitely count.
If you struggle, try turning on Traction Control.
Enjoy!