There are a few main ways you can create a post:
Right-click anywhere on the calendar in order to create a post for that date
Click the "+ Create New Post" button at the bottom of any day of the calendar when hovering over that day
Click the "+ New Post" button at the top of the lefthand sidebar
Use the keyboard shortcut "P" to create a post from any page
From here, fill in any post info like the Post Title, Status, Channel, and Post Date. You can always change this information on the post page as well.
