I don't speak english, so sorry for any grammar error.
I did illustrated to have to write as little as possible. How did no measures have not as I spend a model for you, but here I'll show you how I did.
Step 1: Helmet
I used an old bicycle helmet, they are not so expensive and has the ideal size.
I have not used the helmet like this in the picture, used it reversed because the position was better.
Step 2: Structure
Step 3: Cutting and Pasting
if you use a very large cardboard neck, you can not move your head. look at what height is comfortable and cutting.
Step 4: Ending!
Now just paint! Use as a reference the colors of pepa and don't forget to get around with darker colors: the ears, mouth, nose and eyes
Let dry and ready! your head Peppa is ready!