Once you've selected all these faces it would have been faster to use CtrlE > 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.

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.

Once you've selected all these faces it would have been faster to use CtrlE > 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.

Your result is not perfect but:

• Maybe you should work with a bit less polygons.
• You could have extruded the faces of the body to make the neck then extrude again to make the head. If you want to make the neck more cylindric, select the faces you plan to extrude and press AltShiftS and drag (To Sphere), it will make the selection rounder.

Once you've selected all these faces it would have been faster to use CtrlE > 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.

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.

Once you've selected all these faces it would have been faster to use CtrlE > 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.

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.