Note DAU's website have been updated to end with .edu instead of .mil. Given recent changes to the acquisition policy eco-system, it is recommended to review DoDI 5000.88 dated 18 Nov 2020. It incorporates the current DoD policy for engineering of defense systems, to include systems engineering. 

https://www.esd.whs.mil/Portals/54/Documents/DD/issuances/dodi/500088p.PDF?ver=O8LFc8NzlyJX-SgM2Haalw%3D%3D

The guide for SE is contained in Chapter 3 of the Defense Acquisition Guidebook (DAG), but that is to be updated in the near future to a "more modern set of acquisition guides" per direction from the last Under Secretary of Defense for Acquisition and Sustainment, Ms. Ellen Lord. Go to https://www.dau.edu/tools/dag for the DAG.