There are a few different ways to paint fabric without it getting stiff. One method is to dilute the paint with water, which will make it less viscous and easier to apply. You can also use a fabric medium, which will help the paint to spread more evenly and will keep the fabric from becoming stiff. Another option is to heat the fabric before you start painting it, which will make it more pliable and easier to work with. Finally, you can use a sealant after you've finished painting the fabric, which will help protect it from becoming stiff or brittle over time.