Once you've selected all these faces it would have been faster to use <kbd>Ctrl</kbd><kbd>E</kbd> > *Bridge Edge Loops*, or right click *LoopTools > Bridge* (in that case you need to enable the *LoopTools* addon). Also, I'm not sure why you want your faces to be flat.

[![enter image description here][1]][1]

Your result is not perfect but:
- Maybe you should work with a bit less polygons.
- You could have extruded the body faces to make the neck then extrude again to make the head. 

[![enter image description here][2]][2]

[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/2HMfK.jpg
  [2]: https://i.sstatic.net/GzWdC.jpg
  [3]: https://i.sstatic.net/HFvHf.jpg