Hello and welcome to the forum.
No matter what the layout, I wouldn't be comfortable without the use of a multimeter or at least a test lamp.
If you are employing an inline fuse of your own, then attach it to the high side of the fuse point you choose, either always on or ignition as required. By going high side you're not loading the existing fuse and it won't matter which specific fuse point you choose.
I found this for "add-a-fuse adapter". Is this what you're using?
If so, I would imagine the + goes to the high side and the - to the fused circuit. You can usually tell which is which by removing the original fuse and looking for where there is still voltage (high side). Looking at that, it doesn't appear to include an inline fuse, so you may want to add that at the blue end.