Now I'm finished, I also found this youtube guide, which is better than the ones I used to do my 'surgery'
position of the square block on the power button is a little bit left from the center (phone facing you with screen)
This is a good guide, but indeed Byrn made it better !
Re-assembly can be stressful, especially since the phone will go into 'safe mode' or get stuck into the bios boot screen.
Apparantly, when the 2 volume buttons are not working, or pressed in continuously, this makes the OS go into a frenzy, and you're stuck in safe mode.
I tried to assemble the back cover (with power already on, as then there is no cover button pressing), but it went wrong and eventually my phone got stuck in the boot screen and the power button was also not working anymore.
So I opened the phone (with power on) and removed the battery cable (step 10), which reset my phone again, after which is was booting normally.
Assembly of back cover was finally well achieved by inserting the top-right first to ensure proper alignment to the power and volume buttons.
Be mindful of the small ZIF connector, as when you remove it, it will stick out from the back of the phone and can be easily damaged if you put the phone with its back on the table. Better put it flat again after you loosen the cable
As happened to me; the white part came loose, but I managed to put it back. However, it doesnt have as much r'resistance' as initially. Nevertheless my phone is still working, so probably the small ZIF is only to keep the cable in place. This kept me sweating for quite some time...