Look at old mosaics in books or online for design ideas.
Sketch a design on the back of the paper plate.
Tear out magazine pages that are printed with the colors you would like to use, you can also use colored construction paper.
Cut the pages into small squares, plus a few triangles for the corners (they don’t have to be perfect, variety makes it interesting)
Mix together equal parts white glue and water. Brush a bit of the mixture onto the plate and put the squares into place. After putting the squares into place apply a top coat of the glue mixture to seal it. Repeat this process for the entire mosaic.