Possible sieve designs for concentrating the leaf litter:
The design to the left is fancy and very nice. It is a large sack or net with two hoops. The top hoop just holds the top open to receive leaf liter, the bottom hoop holds a 1/2 inch mesh screen. You placer leaf litter in the top and use the handles attached to the two hoops to shake the liter. The finer material will fall through the screen into the bottom of the sack. The end of the sack is tied with a string, simply untie the string to remove the concentrated leaf liter sample.

The box sieve below is much easier to build, just 4 boards with a screen nailed to the bottom. Place some leaf liter on top of it and shake it over an old sheet. Save the material that falls through for your funnel. Return the larger material that stays in the box to the patch of ground where you collected it.

How deep will you dig?
The surface leaves will be recently fallen and mostly whole leaves. As you pick these up and place them in the sieve, you will notice that the leaves below are moister and more broken up. eventually you will reach soil that has few or no recognizable plant fragments. You can add some of this material to your sieve if you like, but note which layers you included in your sample, because each of these represent a very different environment. Do you think that these layers will have different organisms inhabiting them?