You can select multiple tasks at once by using your keyboard.

Press shift and click two tasks to select them and every task in between them. 

Press cmd/ctrl (Mac and Windows respectively) to click on individual tasks to add them to your selection. 

Selected tasks look like this:

As soon as multiple tasks are selected a menu appears at the top where you can apply an action to all the selected tasks. 

To use the task input shortcuts for your selected tasks, just start typing and you can add any attributes (labels/tags time estimates etc.) to all of them.

TIP: You an also bulk select and edit projects in the Master List.