If you have dumbells or a bench pressing bar for both hands at the same time hold it like when you do a biceps curl BUT DONT MOVE YOU ELBOWS only move your wrists like you would your elbows up and down. But if you really want a workout on your forarms play tennis I played for a few months and after two days of practice my forarms hurt more than they ever did in my entire life and its good cardio. and every time i played tennis hard after that my forarms always hurt, almost to the point where i could'nt move my arms.
I found my forearms get enough workout while doing my ther exercises, the muscles are used for gripping. You cannot make your wrist bigger as there is no muscle present here but wrist curls, hammer curls, reverse grip bicep curls are good for the forearms.
Wrist curls - kneel down at a bench holding a dumbell in each hand. Rest your forearms on the bench, with hands over the edge, palms facing up. Lift the weights by bending your wrist keeping your forearms on the bench. You can try this 'reverse' with palms facing down also.
Hammer curls - stand holding a dumbell in each hand like a hammer, your palm facing in at the thigh with the weights at the front and back of your leg. Keeping your elbows tucked in and without moving your back or upper arm or twisting the dumbell, raise the dumbell in the same manner as a bicep curl.
Reverse grip bicep curls - instead of doing a curl with the palm facing forward at the bottom, rotate your hand around so that the palm is facing behind you at the start and is facing the wall at the top of the bicep curl.
Another is holding a barbell behind your back, arms down with your palms away from you and simply moving your wrists up and down. You can also use dumbells instead of a barbell if you prefer. As was stated earlier you can't make your wrists bigger but you can build your forearms a bit. Forearms like calves are mostly slow twitch muscles so you want to do a rep range around 20 to 30.
Keep Pumping...see what happens
Supinated grip on curls is good for the inner forearms and you can increase the effect by putting particular focus on squeezing that contraction at the top of the movement. Pronated grip will do more for the outer forearm. Since this tends to be a weaker grip, this is a good finishing lift to use a lighter weight and push the fatigue of forearms and biceps.
Keep your body lean, your blood clean and your mind sharp. -Rollins