You just need to remove the cocking handle and lever. Then wack it with a hammer to bow "out". Then just stick it back on and insert the pin. To check it, close the lever slowly and see if you can feel the breach seal touch (it will be bouncy). Keep bending till you feel the chamber hit hard (steel to steel) when the cocking lever is closed then bend it back just a tad. You'll know if you went too far since the cocking lever will not want to shut or it will spring back out when you release the button.
That's the best way to describe it. You want the chamber air port to be up against the barrel very close. Just touching or a couple thousands from touching. Like I said, if you bend too much, the cocking lever will not lock down or it will be hard to do so, so just tap the lever with a hammer a bit till you "back it off" a bit. You will not need any shims after this.
