Making a psi ball is basically an exercise in influencing the motion and quality of psychic energy. Once mastered, the psi ball technique can serve as a basis for most (if not all) psychic abilities.
There is no need for strong, exclusive focus - in this case an open palm is better than a tight fist. If you try to grip the psychic energies too tightly, like sand they will flow away from your hands.
For beginners it is enough if you make the psi ball in your imagination. Just visualize the movements in as much detail as you can. If it's easy to imagine, you are on the right track. Play with the flow of energy; what sounds does the psychic energy give of, what color does it look like, how does it feel to touch?
With enough practice, sooner or later you will overcome the barrier between imagination and perception. This means that you have found the connection between imagination and reality, and have developed a way to directly interact with a level of reality using your consciousness.
It is not strictly important to form a ball of energy - what is important is to see and practice what kind of forms energy can easily assume and how it can change from one form to another.