Either click Songs (Library on the left sidebar) or a specific playlist (if you want the song to list there as well);
click the Pencil (top right) then choose Blank (no barlines) or from one of the templates:
32 Bars AABA
32 Bars ABAC
48 Bars
96 Bars

In the editor panel on the right:
Choose the time signature (it is entered where the cursor is), then enter the Song title, composer, and style.
Choose the key you will be entering the chords in, then click Set.

To enter chords, either
- use the dropdown menu for Letters then again for chord quality options (choosing from this list makes sure the player understands and plays the chords you are entering), or
- use the Mac keyboard to type in the chords (typing can become a quick way to complete a chart)
More information on editing