Choose the candidate from the grid, and hold down the mouse:

The candidate icon appears. This can be dragged up into the booking sheet and hovered over an empty shift:

When the mouse is released, the shift turns yellow indicating that the candidate is assigned, and the following dialogue is displayed:

Answering Yes will add the candidate to any other empty shifts therefore filling the requirement for 1 temp for all shifts in the booking. It may be possible that the candidate can only work a limited number of shifts in which case answering No may be more pertinent.