My part was to bring all the supplies, and some finished examples. I also used pencil to draw an apple shape on the papers beforehand, so all they would need to do was glue on the paper squares (our time was limited, but if you have the time, drawing the apple shape could be an extra step and some kids would probably like drawing their own or using a template to trace around).
colored paper for a background, cut to the small size we wanted (about 7' x 8 1/2"). I offered the kids a choice between four colors-- light blue, dark blue, yellow, and pink. I was kind of surprised when every single girl wanted pink . . . so most of my pink supply was depleted. The boys mostly chose blue, though one brave fellow decided on yellow.
glue and some small plates to put it on, and cottons swabs to use in applying the glue.
paper squares, in various shades of red, pink, and orange-- I did have a small amount of these precut, but mostly just had long narrow strips (cut with a paper cutter) and went around cutting these off into squares with a pair of scissors, to replenish the squares as needed.
some long strips of brown paper (even more narrow) for cutting the apple stems-- I saved this for near the end, when kids were almost finished with their mosaics.
Here is what a finished mosaic looks like:
One thing I did to make some different shades of colors for the squares was to use paint (tempera and also watercolor) and paint some cardstock with reddish paint. This also gave an interesting texture to some of the pieces-- because there were brush stroke lines. It also gave more variation in the color among the squares. Watercolor paint gave me pinkish-red shades; the tempera paint I used was reddish-orange.
In the sample above, some of the squares were cut into rectangles and triangles, to better fit in along the lines.
An apple is a great subject for a fall mosaic, because apples are harvested in fall, and it is a simple roundish shape. I think this could be also be very easily done for pumpkins or jack-o-lanterns.
If you'd like to see some more mosaic ideas, using all kinds of materials, there is an assortment here on this Pinterest mosaics board: