Workbench Flashing - LegacyNsfw/PcmHacks GitHub Wiki
This diagram was originally posted here:
http://www.gmtruckcentral.com/articles/2014/Bench-harness/bench-harness.html
Also note that if a checksum is invalid, the PCM will assume that the flash chip is defective, and will refuse to be reflashed without an extra step - see this YouTube video for details: