What can I do as a parent to help my child?

Play counting games, requiring subtraction, whenever you get the opportunity. 

Should I teach my child my own method?

As long as it doesn't confuse them and you know exactly how it works.
The methods we have chosen to use are ones that we believe will help the child to develop the understanding. Ask your child to explain how their method works; then you will know if they really understand it.
Subtraction is the opposite of addition, although many children find it difficult to grasp the link. 
       11+6=17 so therefore if you subtract 6 from 17 you will go back to the start - 11.

To help develop this understanding, we often talk in our academy of numbers coming in threes.
11 + 6 = 17
6 + 11 = 17
17 - 11 = 6
17 - 6 = 11
What do these methods look like?