While working on my 3x3 double slit method (which is also possible though a bit slower than the standard 2x4) I found a much faster method than the double slit at the cost of an extra screw pump in the lowest aquifer level. I could not find anything similar in the forums so I made a step by step tutorial that you can find
here. I won't post the "3x3 double slit method" because this method rendered it obsolete.
I will call it the "weep holes" method. Although similar in concept to the double slit method, this system has some major differences: It allows the construction of 3x3 staircases (or 3x4), it avoids the problem of job cancellations except in the two lower aquifer lever (which makes it a good choice for multiple aquifer layers), in the lowest aquifer level it uses two pumps instead of one and it is much faster (I went through 3 layers of aquifer in 1 month and 3 weeks).
Same as the double slit method plus an extra screw pump in the lowest aquifer level.
I recommend:
Building walls with blocks, not with raw stone. Have 20 stone/wood blocks ready for building walls before securing a new aquifer level.
Building a screw pump before starting the process and having another one ready when you get to the lowest aquifer level.
It is recommended when embarking on an aquifer to have:
+ At least one proficient carpenter and one axe.
+ At least 2 picks (better to have 3).
+ At least one proficient mason and 15 stones (to produce blocks as fast as possible).
+ Extra food and drink.
If you are using DFHack it is HIGHLY recommended to run autounsuspend: write in the DFHack console "autounsuspend start". This way you can avoid the job cancellations pauses in the two lowest aquifer layers. When finished you can turn it off with "autounsuspend stop".
And finally, do not forget the rest of your fort while digging through the aquifer.
Starting the process - 1 day.
Preparations when a multilayer aquifer is found - 5 days.
Securing an aquifer layer (except the lowest one) - 1 week.
Securing the lowest aquifer layer - 1 month.
So an aquifer of one layer would take 1 month + 1 day (approx) and 7 layers of aquifer would take 1 month (lower layer) + 6 weeks (the other 6 layers) + 5 days + 1 day (approx).
In all the aquifer layers except the two lowest ones
The only disadvantage is that you need to construct 2 extra walls in aquifer tiles (14 instead of 12).
The advantages are that while the slits of the double slit method fill 4 tiles, the "weep holes" fill 10 tiles. This means that every outer wall constructed has an adjacent tile draining water so you get no job cancellation spam and so each level is constructed really fast.
In the second lowest aquifer layer
A clear disadvantage is that you need to construct two extra walls in aquifer tiles (14 instead of 12).
The advantages are that this method uses 6 draining tiles of "weep holes" (instead of 4 that has the double slit method), and instead of having one dwarf working in the 4 tedious outer walls of the center, you can have two builders working on it at the same time.
In the lowest aquifer layer
The obvious disadvantage is that it needs an additional screw pump (and so an additional pump operator) in this layer. But having two screw pumps also allows you to have two builders instead of one working through most of the process, significantly reducing the amount of time invested.
Another advantage is that the weep holes method needs to build the same number of walls (16) than the double slit method despite of its bigger size. Also, it only needs to build two ramps instead of three. Moreover, it does not need a screw pump to be constructed and deconstructed 2 times to build the ramps, nor the construction/deconstruction of stairs/floors (for the pump to work while those ramps are constructed).
Finally, while the double slit method constructs/deconstructs 1 screw pump 4 times (also the stairs/floors in which it works need to be constructed/deconstructed), the weep holes method needs to construct/deconstruct 2 screw pumps only 2 times reducing the amount of time invested (and it does not need to construct/deconstruct their stair/floor tiles).