You will also want to have 4 beacons in place so that your are full covered with Haste 2 no matter where you are in the 64 block radius.
My first witch farm dig out I was able to pull out about 4-5 DCs of coal ores and more stone then I needed. This will produce roughly 150+ DCs of items from stone, ores, etc. Silk is optional but you not have to cook all the cobble to make stone. For fastest results have about 18 Efficiency 5, Unbreaking 3 Silk 1 picks on hand. This is also a good location to bury your establishment chest as it will be hidden under the ground and out of the way. Once you are at the desired level, replace that block with glowstone and a sign that says 'middle' or something. To start off you want to find the middle of your witch farm and dig straight down to the layer you want to stop at. This will give you plenty of room to hide things. On my new farm I will be making the actual floor 2-3 blocks above bedrock.
and made it more difficult to hide things. This did not leave a lot of room for hoppers, item elevators, etc. For my first farm I dug down all the way to the top layer of bedrock.
