Mob Grinders

So, after donating 50 dollars and getting my mob spawner I can't seem to figure out how to set it up so they spawn and fall down into a hole where I can kill them. Can anyone help?

 

This happened on org yesterday... Their mob spawner wouldnt spawn anything :I

 

i can help heres a image of my grinder

 

What does the top look like? What are the dimensions?

 

its a 8x8 Room height 4
Image:

 

Thank you

 

Do you have any redstone wired into that piston or is it just that lever?

 

just a lever