Glue the magnets to the flat surfaces of the rotor core with the letter S facing outside. You have to glue all 4 magnets, the optointerrupter motor will not work with 2. Straighten the T-pin if necessary. You can check it by spinning the rotor between your thumb and index finger. Again, be very careful.
All kits have magnets with the South pole marked.
If you want this side to look better you may cut out the white glossy round labels that are provided and paste them to the marked sides. You may do it before attaching the magnets to the rotor. It is recommended to use regular white glue or a glue stick on the labels for better results. Poke a hole in the center, which is marked by a cross. Slide two sequins as shown below.