Put your five fingers on the keys (c-d-e-f-g-a-b) and press them together. Keep the wrist soft and avoid tension in your arms. Then, while four fingers hold the whole notes, the other finger plays its key in quarter-notes, piano or pp. Do this with each finger, fast if possible. When the finger rises, after holding, keep it allways in touch with its key. Repeat this procedure several times but avoid tension and/or muscular pain. Keep your hand quiet and your wrist loose. First, with one hand. When you feel tired, with the other hand.
In this kind of exercise, the 4º finger is the weaker. So, play 1-4...2-4...3-4...4-5 several times.
Pay attention to play evenly (equal strengh of tone) with all the fingers. And play only with a light touch. Another exercise: allways with the five keys down, play 1-2-3-4-5-4--3-2-1... hearing whether the tones sound equal in force.
Best wishes.
Rui