A magma forge/kiln/glass furnace/smelter/etc. requires one open square beneath it that opens onto lava of at least depth 4 in the Z-level below. Common practice is to make that one square be one of the impassable squares of the workshop you are building. (So those silly dorfs don't fall in.) For magma forges, that's one of the two side squares. For everything else, it's the top center square.
Magma Forge: Everything Else:
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XXX XXXI've never used so many channels so close together, myself. I usually dig out my workshops, figure out what I'm going to put in each row of shops, then dig the laval tubes underneath as needed. To me, the layout Anaris has seems awfully small. I build all my workshops in 5x5 rooms with a door, so I can lock in any insane crafters. I'd make the lava tubes at least twice as long and space them farther apart, even if they aren't used. It's easier to dug out more than you need to start with, than to add more later.