You can do this by going to the "Posts and Comments" area of your Blog Module.
Posts & Comments
Clicking on "Add Post" will take you to an editor where you can create a new post
NOTE: once you save your new post it will be visible (live) on your site
Here is what the create a post page will look like:
You can choose the Author. (note the author is anyone that is setup in the membership module linked to this blog module, and specified in the "General settings" area)