How do you make cotton waterproof?

To make cotton waterproof, you can apply a waterproofing agent such as silicone spray, wax, or a fluoropolymer-based treatment. These substances create a barrier that prevents water from penetrating the fabric while still allowing it to breathe. Another option is to use a waterproofing wash-in product during laundry, which can bond to the fibers. Always follow the manufacturer's instructions for the best results.