What does put the paper to bed mean?

"Put the paper to bed" means to make the final changes in a newspaper and send it to the printers. In the old days of linotypes and the earlier "hot metal" type, the metallic type had to be locked into place for the printing presses, and no changes could be made. If you'd made a mistake, you'd read about it the next morning, because there was NO WAY to change it.

Most newspapers would do an "early edition" that was printed overnight; many papers could make limited changes for the "late edition" later in the morning. And for really urgent breaking news, the paper could do an "EXTRA!!!" edition at any time during the morning.

By noon, it was too late; in any big town or small city, there was normally a competing afternoon newspaper which would be "put to bed" about 2PM and printed in time for an afternoon delivery or for people to purchase and read during the train ride back home.

Now, there isn't any cutoff time - and there's almost never an "afternoon" paper. There's no metallic type, the "paper" is typed on computers and send directly to the presses; there isn't even any time for editors, which we can tell by the astonishing level of errors in modern papers. We get breaking news from the web, or on Twitter, or as text messages on our smartphones.