measure the sises of your existing door (length+width)
Draw your door on a piece of thick card board about 3 or 2 mm thick
on a piece of paper draw the model you will like to have and split the layer s up.
Draw the bottom layer around your door ,then 2nd then 3rd etc.
cut each one out so together it will come to make your model like in the first picture
Next stick them all together and you have a door way
this is how i made all my doors also along with windows + window frames!