From the user menu top right click on Profile>Blog entries.
Alternatively, if the Blog menu block is enabled in the course, click Add a new entry there.
Write your entry and give it a title.
If you want to attach a file, click the Add button to access the File picker to locate a file. Be sure your document is smaller than the maximum attachment size. Alternatively, drag and drop your file into the box provided.
Choose who you wish to publish the entry to i.e. who may see the entry. There are three options:
Yourself i.e. your blog entry is a draft
Anyone on your site
Anyone in the world
Select appropriate official tags for your entry and/or add one or more user defined tags. If you add more then one, they should be comma separated.
Click on the "Save changes" button.
If you click "Add an entry about this course" in the Blog menu block then you will have the option to associate your blog entry with your current course. (Note -your blogs can still be seen elsewhere in Moodle, but this is a way to filter entries about a particular course within the course)
There is also a forum type "Standard forum displayed in a blog-like format" enables teachers to create forums which work as course blogs.
Sites upgrading from 1.9 which previously had blog visibility set to "Users can only see blogs for people who share a course" or "Users can only see blogs for people who share a group" will have blogs converted into blog-like format forums.