Hi
the mesh always stays in the bottom tray, and seems to adding to the confusion of what happens to it
The Mesh is there to prevent the worms falling DOWNWARDS into the sump -therefore it always needs to be in the lowest tray.
therefore to start (when setting up the wormery) you have
sump with 1 tray above (with the mesh at the bottom, coir bedding, worms)
When this tray is nearly full add the next tray on top
then when this tray is nearly full add another tray on top (dependant on how many trays you have)
the worms are surface feeders and will move upwards into the upper trays, leaving the eaten food (vermicompost) in the lower trays.
At some point your bottom / lowest tray (that has the mesh in it) will need to be removed and emptied and replaced back on the top of the pile.
The mesh needs to be replaced into the new BOTTOM tray - simply shift the food/ compost to one side and place it at the bottom of the tray
Continue rotating the trays - always remembering to place the mesh in the bottom tray each time
Hope that makes it clearer
Ron