How is Acerin used for warts?

Acerin liquid is designed more for removing corns and thickened skin. Warts can be removed using e.g. Brodacid (a stronger agent) or freezing preparations (a faster and less burdensome method).

More information:

https://www.gdziepolek.pl/produkty/13867/brodacid-plyn/ulotka

https://www.gdziepolek.pl/produkty/83928/wartix-aerozol/ulotka

Brodacid should be used as follows:

'Lubricate the clean and dry surface of the nipple with the medicinal product using the applicator. After unscrewing the cap and removing the applicator, wait a moment for the excess liquid to drain into the bottle, then apply only to the surface of the nipple, taking care not to get the liquid on healthy skin, mucous membranes or eyes.
After smearing the nipple, wait a while for the liquid to dry in the form of a white film. Apply twice daily to clean and dry skin. Lubricate only the surface of the nipple, depending on the result of treatment, for several days, up to a maximum of 6-8 weeks."

I encourage you to read:

https://www.gdziepolek.pl/artykuly/jak-pozbyc-sie-brodawki-kurzajki