You know, through the border crossings that are all closed. They're just announcing this to claim that they gave people ample warning to leave when inevitably scores of Palestinians end up bombed.

You can't. Isreal knows this. But by saying "We told them to evacuate" they've now provided themselves sufficient cover for everything that happens after. When civilians get slaughtered on mass they'll just keep repeating "They were told evacuate".

Not to mention the old, the sick and the disabled. On top of that the numerous injured from the bombings.

Answer: You don't. Not in 24 hours and especially not when the people in charge don't want the civilians to leave either. This will be used by Israel as a thin justification that whatever they do next did not hit civilians because they asked them to evacuate (the 24h hours part of this ultimatum will be dropped under the table, never to be mentioned again).