To schedule a post, click on the gray clock icon next to the blue "Post Now" button at the bottom of the Post Composer.
You will see a column labeled "Upcoming Scheduled Posts" appear to the right of the post composer. This allows you to easily see what is already scheduled so you can plan accordingly.
The scheduling calendar at the bottom of the post composer will default to today, or you can select a different day in the future.
Type in the hour, minute and AM/PM for your post. Be sure your timezone is correct.
Once you're finished creating your post, click the blue Schedule now button to save your post for later.
Once a post is scheduled for later, you can view it in your "Scheduled Posts" stream, found by clicking the pink Add to Dashboard button. Select the post type and tag (if applicable) and the social accounts you wish to view in the stream.
NOTE: If you want to change the accounts you're monitoring, click the sort and filter icon next to your selected accounts.