Gather as many different kinds of test samples as you can. But limit yourself to small, thin items. For example: aluminum foil, tin-can lid, newspaper, rubber band, leather, paper towel, wood, cotton, nylon, toothpick, broomstraw.
Put an inch of soil in the tray. Spread your samples on the soil, keeping a record of where you put what. Then cover up the samples with another inch of soil, and water the soil enough to make it moist (but don’t get carried away and flood the container). Add a little water every day.
After a month or so, pull out one or two samples to see if enough decomposition has taken place for you to draw some conclusions. If so, carefully remove the soil covering the rest of the samples. And start concluding.