Download the latest version of the Blog App checking that the minimum version of Perch is the same or less than the version of Perch that you are running.

Unzip the download where you will find two folders.

Copy the perch_blog folder into perch/addons/apps/

Add perch_blog to your apps.php file.

When you next log into the Perch admin the Blog install script will run behind the scenes to create the necessary tables. A new blog navigation item will appear in the administration menu.