Here is a screenshot of all items you will need. For this setup I will assume a farm of 5x6, but it can be expanded infinitely.

Quick Notes:
-The Large Container is preferred for more storage
-The cheaper the robot the better

• 24 Floors (any) [Wood Plank Floor pictured]
  
• 6 Conveyor belts (any) [High-Speed Conveyor Belt pictured]
  
• 72 Farming Plots
  
• 30 Walls (any) [Bright Wooden Wall pictured]
  
• 2 Sprinklers (any) [Extensive Sprinkler pictured]
  
• 48 Wood Grained Patterned Wall 1/4th
  
• 2 Robots [Agricultural Robots pictured]
  
• 36 Wood Grained Patterned Slope 1/4th
  
• 6 Multi Sling
  
• 30 Fans
  
• Container (any) [Large Capacity Container pictured]
  
• 1 Artificial Monolith 3x3

To start pick a place for your crops to be collected and put down your Container and surround it will floors for easy access. This needs to be at least 1 block off the ground. From your container place a conveyor belt feeding into it. Next place a floor on either side of your conveyor belt and a second floor touching those. Your setup should look like this:
[floor][floor][ conveyor][floor][floor]
[floor][floor][ container][floor][floor]

Repeat the [floor][floor][ conveyor][floor][floor] until you have 6 rows of them.

Place your farms down on each floor.
Then place a floor above the farming plot and place a farm on this floor. Jump up and move around until it shows you are looking at the floor and not the farming plot. (see example below)
Pick up the floor and the farming plot will fall and stack upon the previous farming plot. Do these exact steps again to get the farming plots to stack 3 tall.

**WARNING** You are able to do this multiple times, but I've found anything more than 3 tall will result in crops getting stuck and or other odd behavior

Repeat this for your entire farm. Stacking the farming plots to be 3 tall, so your 24 floors actually hold 72 farms.

Build walls around the edges of your farming plots. There should be a 1 wall gap height difference. Enclosing your farm should take 22 walls.
The other 8 walls should be used to create a gap over the farmland (see below)
Toggle to Grid Build Mode (pressing "B" on PC). And place your walls a quarter over your third farm.
This creates a space so that your farming robots cannot accidentally kill one another, but they can still harvest the farm.

Place your Wood Grained Patterned Walls 1/4th, 2 spaces away and parallel with your farms. This will prevent the robots from being able to hit and destroy them.
Attach fans to these lower walls. I like to do 1 on each edge of the farming plot and one in the middle (in line with the plant icon). If you ever find your plants aren't being blown onto the conveyors try re-spacing your fans.

Place your Robots, into your farm. The upper walls will keep them inside your farm and everything should be spaced far enough away that they cannot hit or hurt anything.

Use the dumbest, cheapest robots available. They are the best ones. No joke.
The "smarter" the robots get, the worse they perform.
I was able to get this to work with the Agricultural Robots but they still break down. Just use the Rotating Saws Robots and thank yourself later.

The more tasks the robot can perform the longer it takes them to move around and harvest the crops. On top of that, the smartest robots the "Automated Robot" which can plant, water and harvest: Will often get hung up on other crops that it cannot reach. And it apparently can see for over 200 squares away. It has the worst AI too, taking 7-10 seconds per farming plot.
You won't need any of that functionality anyways. Because if your use the robots planting they can only do 1 seed per field.
Anyways rant over.

Place your Sprinkler on top of the Wood Grained Wall 1/4th, at the 3rd farming plot position. Then repeat for the other side.
If you use the Extensive Sprinkler then you can place them every 5 farming plots as they have much larger range.

Go below your farm and find the center of 4 of your fields. Slightly past the center you want to put down your Wood Grain Slope 1/4th. (see picture below)

Place 6 down. 2 slightly past the center, then 2 below that and 2 below that, to create a small ramp. (see below)



From there we are going to stand on the very top of the last 2 ramps and attempt to put down a Multi Sling (see picture to the Left). In order to do this you will have to jump. When you do you will see the sling go briefly blue. Click fast or just click a lot and it will place. You will know if you've got the placement correct, because the sling will stay tilted upwards. If too low, the sling will topple backwards and fall on the ground. Retrieve it and try again.

Setting up your Multi Sling is easy. Most* crops take 2 min 30 seconds to fully grow. Your Robot will take the below amount of seconds per farming plot (not stacked), so 24. Using this you can safely set your Multi Sling to go off every 210 seconds and get a full harvest. Since your Multi Sling is pointed at the exact center of all 4 farming plots, each time it goes off it will seed all 12 plots by using a single seed.

* Tomatoes grow in 1 minute

Alternatively, you can set each Multi Sling to go off at the max (300 seconds). And guarantee a full harvest. Slower but still good.

Place a floor above your 3rd conveyor, then place down your 3x3 Monolith.
Pressing 'E' on the monolith shows its effective range. Make sure your farm is encapsulated. And viola! A fully automated Farming setup. Now just buy seeds from the merchants and throw them into your slings below.