The bill of materials is actually pretty small, so I have to pad this section by actually writing stuff or else it would just consist of two things: 2 liquids and some stones.

You will need 2 different liquids. Usually this will be water (obviously) and either salt water or brine, which can both be gained with some luck from the printer. We only need a few grams, so the 2 tons the printer usually gifts us is more than enough for hundreds of water locks. Don't try to use polluted water, its tendency to emit gasses destroys the lock.

And then some sandstone, granite or whatever else you want to build the bottle emptier and the blocks out of.

Create and place stone blocks and the bottle emptier as shown in this picture:



As you can see, the bottle emptier points to the 2 blocks next to it where the water lock will be created. It is important that the blocks behind the water lock (opposite the bottle empiter, on the "other side" of the lock) are solid, either wait with digging until you're done (as I have done here) or put some sandstone/granite blocks there. Also, it seems that the water lock is more stable if the blocks above and below it are non-natural (i.e. built blocks). Leave one block behind the bottle emptier free, then place a "blocking" block so the water doesn't flow everywhere and you need to mop up 10+ blocks.

Empty the first liquid. It seems to be working better if you start with the salt water or brine and use the water as the second liquid. Make sure that all 3 ground tiles are wet (sometimes, only the tiles where the water lock should be and the tile the bottle emptier stands on get wet, then just mop everything up and start over). It is very important that the tile behind the bottle emptier is wet, because that's the only tile we'll mop up, if it isn't wet, this will not work.



As soon as all 3 tiles are wet, immediately cancel the bottle emptying and mop up the one tile behind the bottle emptier. Make sure you ONLY mop up that one tile.



The trick is that when you mop up a tile, the tile and the tile right next to it get cleaned. So cleaning up that tile next to the emptier will lead to only the one tile where we want the water lock to exist keeping its puddle of water.

Now do the same with the other liquid (usually water).



And again, only mop the tile behind the bottle emptier



You end up with a "wall" of two liquids on top of each other

You're basically done. Before dismantling the bottle emptier, I highly recommend breaking down the wall behind the lock to see if it's stable (sometimes the game remembers that balancing two liquids on top of each other shouldn't happen and they glide off each other). It usually works better if you first remove the upper block, then the lower.



You can now build whatever you want right next to the lock. This is for example pretty much the middle of my base where I lock off the stinky water treatment plant.

Note that this lock is only semi-stable. For example, it reacts very badly if you try to transport liquifiable objects through it, for some odd reason trying to move ice through it makes it collapse. It also doesn't work every time, if it doesn't work for you, keep trying, it sometimes takes a try or three. Make sure you have a wall on the other side of the planned lock, make sure that you only mop up the tile behind the bottle emptier and not the tile under the bottle emptier.

Also, you cannot build in a tile where there is a material that gasses something, in other words, you can't build where there is bleach stone, oxylite (also not in solid form, i.e. on an oxylite block), slime or any other rubble that emits gas. Move that gas emitting material first, then build the lock.