To many among us this is really old news, but.
I've always cringed whenever I see in disassemble instructions that state:
"Press the barrel on a hard surface"
To me, that's just wrong.
There are better ways. By far the easiest has been divulged, if I remember correctly, by Ithacaartist. After checking that chamber is clear, insert an empty magazine pull the action open until it locks, remove magazine and then rotate disassembly lever. It's easy effective and doesn't involve external hard surfaces.
Now if you want to impress your friends, you can do it the old Army way:
First step, hold your Luger with your right hand like this (2 pictures provided)
Second step, (no pictures, sorry) while holding the pistol in step one position, press the barrel back with your left hand.
Third step, making pressure with your index finger on the right toggle knob keep the barrel into retracted position (again 2 pictures)
Fourth step, while maintaining 3rd step rotate down disassembly lever.
You'll find that practice maketh perfect.