Once the roller box is complete you are ready to try the trommel out. Mine fits perfectly on top of my wheelbarrow which is handy so you may want to take your wheelbarrow size into account when planning your dimensions. You could also place the trommel on sawhorses or make permanent legs if you prefer. Just make sure the output side of the trommel is slightly downhill from the input side.
If you have a large quantity of material to screen you may want to find a buddy who will keep turning the drum while you continue shoveling jmaterial. If you are working alone then you will have to load a couple shovelfuls into the drum and then give it a few turns yourself before loading more compost. Or if you are really mechanically inclined you could figure out how to hook up an electric motor like this guy did