To make fake flowers look old, you can use a process known as distressing. This involves using different materials and techniques to give the flowers an aged or vintage appearance. The first step is to clean the flowers thoroughly to ensure that they are free from dust and dirt. Next, you can use a paintbrush to apply a layer of brown or grey acrylic paint to the flowers. This will give them an aged look. To make the effect more realistic, you can use a sponge to dab on the paint, creating a mottled effect.
Another technique is to use a spray bottle filled with a mixture of water and brown paint. Spraying this onto the flowers will create a more subtle, weathered look. If you want to give the flowers a more textured, grungy appearance, you can sprinkle them with a little sand or dirt while the paint is still wet. After the paint has dried, you can use a dry brush to lightly brush off the excess sand or dirt.
Remember, the key to making fake flowers look old is to be creative and experiment with different techniques until you achieve the desired effect. It's also important to let each layer of paint dry fully before applying the next, to prevent the colors from blending together and creating a muddy effect.