After installing this author box plugin, at the bottom of your post edit screen, you will always see the Meta Block of Author Box Pro.
On the right side of the Meta Block, select the author, and on the left side, write the contribution of this author.
After assigning a guest author or contributor, the post shows. You can assign multiple guest members in a single post. For this, you have to click on the “Add Another” button.
If you don’t have any authors on your site, you have to add authors first.