It is quite easy, you just need to ensure the rim slides under the extractor claw before closing the bolt. I load the magazine with 5 rounds(or 3 in the case of the 500 Jeff), I then place the round I would like to chamber on top of these rounds. Using my left hand around the bottom of the magazine plate so that your fingers come around the stock I pull this round down into the magazine(there is enough space to do that), you then slowly push the bolt forward and visually check that the rim slips below the extractor claw as you push forward on the bolt.
You can also use the right hand to push the round down and the left to operate the bolt, whichever is easier.
This may sound complicated but is second nature for us who use these rifles on a regular basis.
It is very important to ensure the rim slips underneath the extractor before chambering the round in order to avoid damage to the extractor.
I hope that helps to explain it.
I would also not part with my ZKK 602 actioned rifles, they are just too good and too reliable.