We're trying to flatten chicken wire (for our raised bed fence) that came in a roll originally. Any tips?
You'll never get it completely flat before you use it, though it will tend to flatten over time.
To get most of the curl out, unroll it "upside down" so that the wire on the end bites into the ground. Put a couple of cement blocks or something heavy on that end. Roll out the rest of the fence, putting weights on it wherever it wants to curl back up. Leave it for a couple of days.
Or, don't bother trying to flatten it before you use it. The curl only has enough strength to be annoying, not enough to undo itself from where you attach it. Drive in stakes where you want the perimeter of the fence. Unroll the first bit of fence and attach it to the first stake. Stretch the fence as you unroll it and tie it to the next stake. Continue, stretching the fence so it stays tight and flat, and making sure to attach it to the stakes as you go.