There are two screws holding the recoil pad on (look for tiny +s in the rubber and put a little soap on the screwdriver). After removing the recoil pad, you should be able to see a nut on the stock bolt. I have no idea what it is on a Stoeger, but on my Beretta gas gun it is a 13mm nut). Once undone, you should be able to pull the stock.
The owner's manual is available free online - you may want to print a copy for yourself.