First the main board must be removed from the 1227747 or 1228746 ECM:

WARNING: Remove the main board from the ECM ONLY on an anti-static surface or the internal components may be damaged by electrostatic discharge!

Then the factory connector must be removed from the main board by de-soldering the pins:

A word of caution: If you are not experienced or comfortable with de-soldering methods then you should NOT attempt to remove the factory connector!   The main board may be easily damaged by improper de-soldering methods.  An installation service will be available for those that are not comfortable with de-soldering or do not have access to someone qualified to remove the factory connector.  See "Ordering"

The Adapter is then mounted in place to the two tabs (that GM was nice enough to provide) using two 6-32 screws and two #6 x 1/4" spacers:

The two mounting tabs will be un-threaded.  Using a self-tapping steel 6-32 screw or a 6-32 tap will easily thread the mounting tabs. 

There are three options for mounting the Harness 
Adapter Module to the ECM chassis rails:

1) By using a 6-32 hex-head capscrew and a small wrench.  

2) The main board may be totally separated from the ECM chassis rails in order to get a screwdriver to the adapter mounting screws (which means disassembling the injector drivers from the heat-sink).  With this method the thermal insulator between the 4 power transistor tabs and the chassis heat sink should be replaced to avoid an electrical short.

3) Two holes may be drilled in the ECM main board to allow access to the mounting screws from the bottom (use the holes in the mounting tabs as a guide for the drill).