Here you can see the process of scarfing in. The corner post of a french 3 door bifold set that divides the living room and the outside. This would not be cheap to replace. The customer was quoted £7000 for
the replacement of the frame and doors by a builder / installer. I did
it for less than £500
Rot and my markings of what needs to be cut out. 
Cutting out of marked lines 
Remove the piece 
A template is made 
The scarf glued and screwed 
The finished piece
Image 1. Shows the rot and my markings of what needs to be cut out.
Image 2. The cutting out of marked lines.
Image 3. The removed piece.
Image 4. A template is made to fit exactly into the cut out. This is
then transferred onto the timber that will be used. The scarf piece.
Image 5. The scarf. Glued and screwed with stainless steel screws and
timber plugged.
Image 6. The Finished piece with primer and undercoat.
Carpentry Services Southampton 
Nice Post