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)

Useful editor key commands
cmd-N (new song, template Blank)
cmd-B add left barline
Double-click inside a measure to play from that measure (click on the chart to stop play)
Return - to enter chord, move to the next space
Measure repeat - %
N.C. - n

(Additional key commands on the right of the menu entries and hover mouse over the editor buttons for tool tips)

Some symbol buttons toggle on/off/on... to either input or delete, like END, Segno, Coda symbol, Rehearsal letters.
[Barlines also toggle input/delete/input... (but the singles and doubles have right and left, so if it does not delete, click in the space on the other side of the barline and use the other barline equivalent button to delete instead)]

* use ^ for maj, hyphen for minor (-), lowercase letter o for dim, option-o for half dim (depending on keyboard language settings) (ø)