Determine extension concept based on instructional needs analysis, learning activity design, etc. Ideally this should be done at the same time as the overall book content design, not afterwards (i.e., not as a "bolted on" approach rather than integrated approach).
Determine if the extension is at PeBL Tier 1, 2, or 3. See Architecture section of the spec for a description of tiers. The tier level determines many requirements for supporting infrastructure, that you will need to research for feasibility.
Look in open source PeBL extension library to see if extension exists, or an existing one could be used as a template and modified to address the requirement.
Storyboard how the extension will appear to the user, using a particular sample context in the book (the extension may of course appear in many locations, but pick an archetypal use case).
Test the extension (both unit and integration testing). This may require a testing sandbox configured with the target infrastructure (i.e., learning ecosystem).
Specify a shortcode for the extension to streamline the production process, if desired.
If you are building many extensions, it is recommended that you catalogue the extension in the book design document using the fields used to describe extensions in the PeBL extension library.
If your extension is open source and can be shared, please post it to the PeBL forum and/or initiate a pull request on Github to add it to the open source PeBL extension library.