Your clients can create new posts in Social Marketing by taking the following steps.
How to compose a post
First, access Businesses > Accounts and click on the Social Marketing icon for the account you wish to create a post for. Once you're in the Social Marketing product, click the new post icon, located in the bottom right hand corner of the screen:
Once you've clicked on the icon, you can then use the composer to enter text and links. Once you've finished adding your content, you can either click Post Now in the bottom right, or you can click the downward arrow which will allow you to Schedule the post for a later time:
Note that you can also use the icons in the bottom left hand corner of the composer in the image above to upload a photo, shorten a link, create a coupon, or add interesting content (which will be pulled from the Content tab in Social Marketing.)