A product strategy specification (PSS) is a document that captures a design project's goals and intentions.
You need to document the strategy you've developed for your design project. You need it to be concise and precise, and not represent decisions you are unsure of or have not yet made.
Create a product strategy specification.
Use the documentation you have gathered from the previous steps of the project initialization stage to develop a document.
A PSS is just a report that contains all the information needed to describe the work you've done on product strategy, and justifies all your strategic decisions. Generally, this involves documenting all the work you've done to this point. Every step in the design roadmap should be described well enough that a reader of your design work will understand exactly what you're trying to achieve.
The details of a PSS will vary from organization to organization and from sector to sector. For your purposes, refer to the appropriate report template, which will have the pertinent section headings expected for your PSS. Your job is to add the text for each section heading.