What I do is what Sutremaine said. To elaborate on that process, first build two pump stacks, one for water and one for lava, a z-level higher than you want to the final product to be. You can look pump stacks up on the wiki if you don't know what they are.This can be done concurrently with building the mold walls, but you should have both pump stacks up to start with, since you couldn't do even a single level without them both anyway. Don't forget to hook them to levers, because you'll be turning them off and on a lot. If you want an extra degree of safety, you can put a magma-safe floodgate on each level to let out excess. This won't be necessary if you're paying attention the whole time, but since you're never done this before it might be a good idea anyway. Note that venting excess fluids, especially lava, will make trouble for whatever's standing under and around the floodgate.
Once you've got it all that stuff set up, basically do what Sutremaine said. Only point of interest: Try and shut the pumps off when things would even out to around 4 depth. You don't need the full 7, and if you try for it, you're likely to get water in more than one z, which can cause problems and would require you to vent excess.
Once you've got all the levels done, just strip away the walls and the pump stacks (though depending on how you built them, you may want to leave the lower levels intact).
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Topic: I need a block of obsidian. (Read 1605 times)