using magnetic foil: You need magnetic foil available online, scissors, photos, adhesive tape or double-sided tape.
using clay: Use clay, shape the magnet, and then 'bake' it in the oven for about 20 minutes at a temperature of 100-140 degrees Celsius.
with magnetic sheets: Decorate one side of the sheet, color it, and then cut out in the chosen shapes.
using small objects: Glue small objects to the flat side of a magnet using adhesive.
using photos: Gather self-adhesive magnetic paper, photographic paper, scissors, and glue. Edit, cut out, and attach the photos to the magnet.