There is a very effective way, that is, lift your arm backwards, take a bottle full of water in your backhand and lift it up and down. The bottle should be behind your head and keep it vertical. Don't use too much force on your wrist, just hold it until you feel pain on the outside of your arm. You can clearly feel the muscles exerting force.

This method is used by boys to practice muscles, and it is also suitable for girls because the muscles there are difficult to practice. Girls need dumbbells. You can try. I tried, and it worked. I hope I can help you!