You can initiate new mappings via the following methods :
Just drag your source field to the target field. A simple mapping will be defined. This mapping states that the target field must be filled with the value that was in the source field. You can then update the formula (see the section about how to edit formulas). When you are dragging your source field, all compatible target fields are highlighted.
New mapping (drag-and-drop)
Select your target field, then start typing your formula in the formula bar (see the section about how to edit formulas). When you need source fields in your formula, you can just drag-and-drop them in the formula bar.
New mapping (via formula bar)