Added the genuine article (Marshall Brass Extend-a-stay) here is the procedure I followed.
1: Turn off gas valve on tank
2: Remove male POL fitting from vlave's female POL fitting
3: Move regulator to the right temporarally
4: Screw male fitting on Extend-a-stay into valve
5: Screw original male fitting into female fitting on Extend-a-stay (moving regulator as needed)
6: I did some work to protect the regulator, this may or may not be needed.
7: Open gas valve and test for leaks (NONE FOUND).
That's the whole job.
NOTE: If, in the past, you have had a travel trailer with removable tanks, and you have removed them for re-fill, then you have done everything you need to do to install the Extend-a-stay.. The process is identical to taking a portable tank for refill. (Save you do not move the tank).