1. Start by reading the documentation (link above). Things to look at:

-- creating plugins

-- understanding elgg view system

-- entity types and subtypes

2. Create your plugin. You will need:

-- start.php

-- manifest.xml

-- views/default/forms/{newsubtype}/add.php

-- actions/{newsubtype}/add.php

3. Register an action in your start.php

4. Create a view for your form, register a page_handler if necessary

5. Your action file can assign a new subtype to a new entity

6. Blog plugin is one the finest examples in Elgg, it is written with best practices in mind. Break the code apart and understand what's happening behind the scenes. You will then get into it.

7. Take a look at the pages I wrote: http://community.elgg.org/pg/pages/view/830173/tricks-for-elgg-newbies