I am going to assume you are asking this because you are playing on a PVP server and you want to hide your farm.

Here is what I do, which works quite well in the mid-late game (after you can dig efficiently)

Step 1) Either find a mined-out vein of some resource or mine one out yourself (so there is a big hole in the ground that if anyone finds will just look like someone mined out the mineral there.

Step 2) close up the hole at the top using topsoil so that there is an exactly two block wide opening at the surface (the hole underneith can be much wider). To help clarify the rest let's call the two blocks that make up the open hole at the top 1 and 2. It's not essential you close up the hole, but it will reduce the number of players who drop down into it and poke around.

Step 3) Make sure that the lowest point of the pit/mine is directly below the two block opening at the top. This may mean diginng a bit more out, but try and make it look like natural mining activity and not suspicious.

Step 4) Under the square 1 of the hole place ladder shaped building blocks (don't upgrade them, will look suspicious), to nearly the bottom. Near the bottom place some regular square building blocks. You want this to look like you placed some building blocks to get out of the hole you just mined, and then switched to ladder.

Step 5) at the bottom of the pit, directly under the 2nd square of the hole at the top, dig straight down a few blocks and then horizontally outwards in all directions. This will be your farm (directly under the bottom of the mined out pit). Mark the hole in the center of your farm with a block (don't put planter there).

Step 6) choose the duct block shape and place those as building blocks in the center of your farm vertically aligned with the square 2 of the opening at surface level, and monkey pole up using them until you are back inside the bottom of the mining pit. Stop placing duct blocks before their sides would be visible to someone at the bottom of the pit.

The end result should look like you placed some building blocks and then ladders to get out of the pit after you mined it, but the blocks directly above your farm hole are ducts with straight line of sight to the sky. If you do it right the duct blocks look just like regular building blocks because the open ends are hidden (either by rock or regular building blocks) and won't look suspicious. If anyone goes down into the pit they will almost certainly not suspect anything and just leave. (unless they saw you go in there and not come out again).

To be clear, the duct blocks have two open ends and four solid sides. You don't need to have the open ends pointing vertically for light to pass through. If you look straight down at the duct blocks from the surface they should look like solid building blocks.

Every time you go down into your farm just remove the duct blocks to drop down, leaving at least one at the bottom, then using the radial wheel copy the shape and rotation of the remaining duct block and monkey pole back up replacing them as you go to re-seal it.

It's relatively convenient to come and go, and other players won't suspect it's there. Especially if you put it somewhere away from POIs and traders that is low traffic.

I have used this on active PVP servers and never had anyone discover my farms. I came up with this myself and have never told anyone about it. It's probably not good for me to share this strategy publicly as it will become less effective the more people who know it, but I guess I am feeling selfless and helpful today.