How do I replace the serpentine belt on cummings 350 isc

Not sure there is a "secret" to adjusting the A/C belt. If tension was proper before removing it to replace the serpentine belt, use a piece of tape above the adjustment area to mark where you want to return it.

With just moderate tension on the bolts securing the compressor, use whatever means you have to return it to its original position. I use water pump pliers. But, a couple of screwdrivers, bolts, etc in the adjustment area will allow you to pry it back to its proper position.