When making a head of a toy, we are usually told to start by sewing the head sides together, from the tip of the nose down to the bottom of the neck and then insert the head gusset.  If you have trouble inserting the head gusset centrally, try changing the order of what you sew first!  Try sewing the head gusset to one head side, from the tip of the nose to the back of the neck first.  Then repeat from the tip of the nose to the back of the neck on the other head side.  Finally, sew from the tip of the nose to the bottom of the neck. 

We have found this works well and helps to get the head gusset central!