Upload the directory "simple-post-template" to the /wp-content/plugins/ directory
Activate the plugin through the 'Plugins' menu in WordPress
Go to Settings -> Simple Post Template and enter your desired title, body and/or excerpt for new posts. (Feel free to use any combination of HTML and PHP.)
If you select "Auto fill new posts with this template." all new posts will be created with your template. If you uncheck this, the new posts screen will have a new sidebar widget with a button allowing you to apply the template.
There's nothing else to do. Enjoy! (unless you are upgrading, in which case, see below)