My only advice is get training before undertaking this ceremony for others as it is a beautiful service to offer someone but requires much preparation and work on the self so that when you step into the other's energy flow and seek their soul parts you do so without ego needs of your own that might take power from them in the process. No point retrieving soul if you're taking power in the process.
Sandra teaches a generic method which is proven safe and containes core elements that run through most native traditions, indeed she is someone native practitioners go to for training if their own nation has nobody left with the teaching through their own elders. But she does warn against just reading the book and doing the ritual without practice under guidance. The client is in the room physically, or energetically in the case of distant work, and it has to be safe and ethical for them and it has to be all about their results, not the practitioner's journey experience - she's also developed certain aspects of the ritual since writing Soul Retrieval and the later knowledge is based on her immense experience, not changing the ceremony but ways to support integration by the client.
After several years doing it her classic way I evolved my own unique style as we all must, but there are certain elements of the teaching which remain core to the way I do any shamanic ceremony. I learned the method about 2006 on a week intensive training run in Scotland at Lendrick Lodge, where she comes occasionally to share her Medicine for the Earth training (she's coming this summer) but she has authorised trainers in the UK and I've supported Lendrick's on many occasions. I would not teach her method and am authorised to do the work but not train others, just to be clear, and it's rare that I undertake this particular ritual these days for various reasons, so I'm not expressing my opinion as someone marketing myself
But to evidence the importance of preparation and training, she insists on doing either her Shaman's Path (with an authorised trainer) or Harner's Way of the Shaman for core journeying,, divination and power retrieval work AND getting clear success at this level with case studies, prior to accepting anyone for soul retrieval training, and both those plus approved case studies on SR (including the ways your journey is worded to the client) before embarking on extraction which can have more serious consequences if not done cleanly and correctly, so without wanting to in any way put in a negative on what sounds like effective work, I would really say do the training before taking on the retrieval work for anyone but yourself.
Just a tiny example to think over - if you see a part of yourself that is lost and not ready to come back, you may have insight as to where it is and what needs to be done, just seeing it will remind you and allow the energy to move where needed in order to become ready for reintegration becaues at some level you were ready to see that piece and do the work. But to be told by someone who you have given power to in order to retrieve your soul for you, that it wasn't wanting to come back to you, imagine how a vulnerable person might receive that news and what internal destructive patterns might take a hold on that.
I've seen the results of how unskilful practices/sharing can cause pain to someone because they believed themselves unworthy of being whole and took that evidence to feed the negativity without realising. I've had powerstealing done to me by well meaning trainees who got lost in the need to share the entirety of their journey verbatim, whether to prove how good they were or to avoid missing something crucial in anxiety or even to play unconscious power games, and despite my experience and clarity it is a hard thing to find a way back from because you are opening yourself to that person and they are lovely people and they don't mean it but it is like planting a time bomb inside. Someone that comes in desperate need and without the understanding I had could suffer immensely from such an experience, however little they realised at the time that it had even been planted as a seed within them. I cleared one such intrusion from myself during a training that happened about 2-3 years ago but can still quote, almost verbatim, every part of the practitioner's journey with the 4 ways she unknowingly created damage and I can't actually recall much about the end where she said the one thing that she actually should have said without all the stories that ruined that gift. So I know there is still a hook in there somewhere that has yet to heal and I can't work with that person now without wincing and working through my remaining distrust to do so, though I did address the skills issue with her on the day and know she took it on board with the integrity and courage required of those trainings. Anyway, I am saying too much perhaps for the query but it is a matter close to my heart not to take on ceremonies without direct training from whatever lineage you choose (assuming not born into one).