Button with metal loop shank.
Step 1: Cover the top of the button with electrical tape: This stops the shank wiggling about all over the place.
Step 2: Cover the Loop with electrical tape to keep the head from wiggling. (Again might not be required if the loop fits snuggly inside the neck)
Step 3: Insert into the Neck and insert the metal pin to hold in place. (It can be a little tricky to get the pin back in, patcience is the key. A thick needle or other object usually helps to push the pin in or to check the holes are aligned.)
Step 4: Slide inside the head. (For some of my newer dolls, loosening or removing the eye-mechanism was needed for this step, remember to tighten it again, if you had to loosen it.)
Step 5: Put the Doll back together and you're done!
Advantages of this method is that the metal shank on the buttons is a lot sturdier than the obitsu neck pegs, I have had my dolls on these for almost 2 years and none of them have needed altering or fixing, whereas a lot of my obitsu neck pegs have snapped from the weight of the dolls head.
Disadvantages of this method would be if the loop needs bent with the pliers to fit and this can require a lot of strength and patience and it is very easy to bend too large or too small.
Buying the buttons: I found these in my Grandmother's old sewing kit but you can buy Similar buttons here