How to op yourself and turn on the whitelist on a Minecraft server

Type the commands in the GamePanel console, without the /: op YourUsername to become an operator, whitelist on then whitelist add Username to only allow your players.

GamePanelConsole, server started.
op YourUsername: in-game admin rights.
whitelist on: turns on the whitelist.
whitelist add Username for each allowed player, whitelist remove Username to remove one.
GamePanel console after op Steve, whitelist on and whitelist add Alex
deop Username removes operator rights, whitelist list shows the allowed players, whitelist off turns the whitelist off.

Common errors

"You are not white-listed on this server!": check with whitelist list that the username is there, spelled correctly.
A player added before switching to cracked mode no longer gets in: their identifier (UUID) changed. Remove them, then add them again.
A player already online stays after whitelist on: set enforce-whitelist=true in server.properties, and players missing from the list are disconnected each time the list is reloaded (whitelist reload).

Both lists are saved in ops.json and whitelist.json, at the root of the server. After editing whitelist.json by hand, type whitelist reload.