Smooth your adhesive stencils down on your wood discs. Make sure they are really smooth, so no paint creeps out from under the stencils - make sure you use the smoothest side of the wood discs.
Use your spouncers to paint the stencils. The key to getting clean lines is to use a little bit of paint at a time and keep going over and over the area, gently.
Once your slices are dry, turn the wood discs over and hot glue a few magnets to the back. I'd do two so that they don't spin around.
Video
Notes
If you are wondering how I did two colors, I did a portion of the stencil in one color, peeled off the stencil and let the paint dry. Then I came back and did the other color later.