What you'll need:
-Red, Black, and White clay
-Toothpicks (or other tool to mold the clay)

Step 1: Molding

Start by taking your different color clay and molding/roll them into balls, except the black you will want to mold that into a circle. then flatten one side of red and white sphere (you can use the toothpicks to do this easier)
Make sure you safe a little of the white clay for later.

Step 2: Putting It Together

Stack the there pieces so red is on top, black is in the middle, and white is on the bottom. Roll the stack gently in your palms and the colors should stick without distorting the figure. Finally put a small white circle on the black to look like the button on a actual pokeball.
Note that i squared my white bottom so that i could stand up straight