saul, you have to turn the bars when you get to the start of the curved parts. go slowly and trial and error it. Yes, prying the bit open a bit with a screwdriver helps, but its the turny turny in the right direction/position that is the trick.

I havent done it in years, but certainly recall wondering how the .... to do it the first time, and getting the bars jammed a bit, until I thought things through properly and figured it out as I went.

good luck, even if someone else shows you, it will be a skill to learn.
On one of my old bikes about 30 years ago, I could have done with a stem a bit shorter or taller, but just couldnt be bothered screwing around with removing the tape, the brakes and messing getting the bars out again, so just left it and lived with it.
Newer stems are so much easier!