Get Waterproof Spray:
Waterproof sprays are easily available in the local market. Choose one which is made for the type of fabric you want to make water resistant. There are different types of spray for different materials.
Apply it Across the Cloth:
Hang the clothe in a hanger so that you can spray on all sides. The clothes should be totally dry. Spray on an interior small patch of the cloth to check for color fade. If you notice any color fade it is on you to decide whether you want to continue.
Spray on all sides following the instructions on the spray label. Ideally you should keep a distance of 8 inches between yourself and the spray so that you do not inhale the chemicals.
Give it Some Time:
Now let it be there or hang it somewhere else for getting dry.
Do a water resistance check by sprinkling water over the cloth and checking whether water is absorbed or not. You might have to re-spray again to increase the resistance. Do remember that most of time clothes lose their water resistant properties after a wash. So you might have to re-spray if you want to make them water resistant again after a wash.