We ordered this cat flap to install into our PVC back door - putting it in was fairly straightforward with a hacksaw and drill - it took about half an hour and whilst I could have done it myself, I was grateful for my husband's help, as it takes a bit of confidence in sawing and drilling through PVC. There is an enclosed guide in the box which you can actually tape onto your door to help you do it properly.
Whilst the item does have different ways to organise the locking mechanism, I decided to keep it open all the time, because my cats needed to learn to use it and I worried they would get confused if in the early stages, sometimes it was locked. I also found that the magnetic strip which makes the flap less drafty, was too firm for my nervous cats, so I just peeled it off (it's easy to put on and off). This means there is a little draft coming through the gap that has left, but now that my cats are happily using the flap with no resistance, I will probably put the magnet back on.