So, Eva 01 volunteered to show this process even though the last one didn't went quite well...
...so this MP Eva will help us get his neck... Separated...
...easy on him...
...you can see now, every part of the neck can be separated...
...now, bend one side of the joint to max. and align the backside of the front piece of the neck with it...
...now, just slide the neck front up the joint and push it so that it ends parallel with the joint's length...
...put the joint in the body and move it so that you can check it worked, if it did, you'll know XD
Ta-da!