Get that double-jump as intuitively as possible. There are no special ways of timing your jumps, simply hit 'Shift' in mid-air like you hit the space bar to jump initially. I know what you're thinking: "Woah, hold the phone, then you'll hit the ground and take damage! This is terrible! I hate you!" No worries, holding shift again after you've hit it to jump will cause you to attempt to roll with the fall. The percentage of success is based on how far you fall, so you can't abuse the privilege! Don't want to hold shift? Set the Auto-Roll config option to true. Each player can only use the double jump once while in mid-air, but if you want to limit the use further, you can have double jumping expend hunger or an item component, both of which are configurable.
Permissions
Commands
Config