The first useful wet practice lap tells you where the car feels different. Pick a car and track you already know, check that your simulator supports wet running for that combination and set up a solo session. If the sim offers a choice of wet-weather tires or settings for the car, check its guidance before you drive.
Find a braking point you can repeat
Start with a corner that has clear distance boards. Pick an earlier marker than you would use in the dry, then build brake pressure smoothly while the car is straight. Pay attention to how readily the car slows and whether you need to release pressure early to keep it settled on turn-in. Repeat until you have a comfortable reference. Then choose a second corner where the approach or the surface looks different.
Try another line under the same conditions
Take a few laps on a conservative line and then experiment with a wider entry or a different apex. Try to hold the same approach speed so you can feel the effect of the path itself. Keep the experiment on track and back off if visibility or standing water makes the line hard to judge. For an example of how changing water levels can reward alternate lines in a simulator, see iRacing's weather guide.
Finish with a short run in changing conditions if your sim offers them. Write down which marker or line worked, what you noticed when grip changed and what you'd try first in traffic. Next time, use the same one-corner drill at a turn you haven't practiced in the wet.