I found the instructions with the video tutorial link very easy to follow. I’m so glad I added it to my first Portsmith.
I added both the lining and zipper extension to my Portsmith bag. This was my first recessed zipper and the instruction were very clear. The extension lays flat when unzipped, you hardly know it is there. It does lie lower than I expected. I had added an extra zipper pocket and open pocket on the sides of my lining and in order to access them I have to lift the extension up, so not as easy to access quickly. But the bonus is they are pretty much hidden and I think of what I put in the zipper pocket as much more secure. I think I get much more use out of my Portsmith with the additions. Love love love the result.