Pick a level
Choose the level you want to run. The higher the level, the harder the dungeon.
Run a dungeon with a higher level, extra affixes, and a timer.
Mythic+ begins when your group brings a Keystone into a supported dungeon. Choose the level, read the affixes, and make sure everyone knows the plan before you start.
Choose the level you want to run. The higher the level, the harder the dungeon.
Look at the affixes for that level before you commit. They tell you what extra rules will be active inside the dungeon.
Buy your Mythic Keystone, form your group, and enter the dungeon you want to run.
The group leader uses the Keystone inside the dungeon. Ten seconds later, the Mythic+ run begins.
Bring the Keystone with you and have the group leader use it once everyone is inside and ready. The dungeon changes to Mythic+ after the short ten-second start window.

Affixes change how the dungeon plays. They can make enemies more dangerous or change what your group needs to pay attention to.
Read the affixes before you start. A quick look at the rules can save a very long walk back.
The timer starts when the dungeon becomes Mythic+, after the ten-second start window. If you finish before time runs out, you receive the reward for the run.
If time runs out, you can still finish the dungeon. You just do not receive the timed reward.
Mythic+ rewards come from the level you chose. A successful timed run can also give everyone in the group another Keystone, so one good run can lead into the next.
The group leader uses it inside the dungeon.
Yes. You can continue and finish the dungeon, but the timed reward is lost.
Check the level, read the affixes, and make sure your group is ready before the Keystone is used.
Choose a level, read the affixes, and take the Keystone into a supported dungeon.