This video was the one that was most useful. He showed me that its just a case of overlaying an already made clip of film burn and reducing the opacity of the burn to allow the main shot to show through.
So I used his link he shows to get free film burns to use, I then found a lot more so that we are not repeating the same film burns throughout our production. This is an example of one.
The two shots to the left show how film burn can be applied to a normal shot.