Hi!
Not quite sure about what exactly are your doubts regarding the ritual, so thought about directing you to a post that was helpful for me. @Norski seems to like this book very much and his posts are always great, so I would recommend you take a look at it if you haven’t done so (sorry I have no idea how to link it properly, hope you get it😅)
Apart from that, I would totally recommend you make a cheat sheet while reading the book. DoM is great but I have the impression the info for the complete ritual is slightly scattered and you need to read with attention to get it all fine.
What I did:
Opened a doc in the computer, wrote in bold what I have to say, in italic what I have to feel/think/look at during the ritual… made a detailed structure for the whole ritual, then I’ll be changing the details of Shem angels, petition etc for each demon, but the structure remains. It helps a lot to memorize the ritual. There’s a video in YouTube from Jareth Tempest, going about it, I would not recommend it as the sole source for this ritual of course, but as some help to study or remember it.
Best of luck!!