How to uncup live edge wood

When you screw it down to your table aprons or supports that should flatten it out. Don’t forget expansion and contraction and use table clips/ grooves so the wood can move across the grain.

What Steve66 said

It’s a slab that I am attaching hairpin legs to. I used a small steam humidifier and put it under the cupped area (it was mostly cupped at one end). I left it there about an hour and it flattened out pretty nicely. It was approx 1" cupped (22"wide at that point). Now it is almost completely flat. I put some weight on it while it drys out. We’ll see what happens when i take the weight off… I tried to attach a pic but it wouldn’t upload.

That’s an awfully thin board to work with for a top unless it’s perfect.
You can do a plunge slideing dovetail across the grain on the underside.
Install segmented dovetail blocks spaced out underneath and then attach a “stiff back” piece to the dovetailed blocks to flatten out the top.
The plunge method will allow the dovetail groove to be hidden from the edge.
I like to wax the dovetail groove to facilitate expansion and contraction.

Also, if you go through all that effort, select some nice quarter sawn material for the “stiff back” pieces so that they will not have a tendency to bow the same direction as your top cups
Good luck and post a pic!