To ensure the longevity of the print on the black "Thor instead of Peter" T-shirt made of 100% cotton, it should be turned inside out before washing and washed on a gentle cycle at a maximum of 30°C. It is recommended to avoid fabric softener, bleach, and chemical cleaning agents, and to use a mild detergent. The shirt should be air-dried, as the heat of a tumble dryer can damage the design, and should only be ironed inside out on a low heat setting to avoid direct contact between the iron and the print.