For the bulk of my life, not only did I not believe in spirit guides, I actively disbelieved in them. But I had a few experiences that forced me to change my opinion on the subject. I now believe. In my experience (plus what I've learned elsewhere), the influence of a "spirit guide" manifests in precognitive warnings, hints, suggestions and the like. For most, this information is veiled and disguised so that it blends seamlessly with our usual interior dialog. It rarely intrusive.
Because of this, it's easy to dismiss the whole subject. In fact, who's to say that all these promptings aren't manifestations of our subconscious, or "God," or even genuine precognition? I suppose that there really isn't any way to prove that spirit guides are behind all this--but I like to explain unknowns with the lowest common denominator. A certain portion of "spirit guide" information involves premonition of events that are impossible for the conscious ego to foresee. It also involves complicated arrangements of future timelines involving many people. It's just easier for me to imagine that there's a "someone" coordinating this behind the scenes.
So, how do spirit guides work (hypothetically speaking)? Well, based on everything that I've read, this depends on the guide, on you, on where you are in life, and what you need to achieve at your present stage. We all have a deadline--literally. There are things we must learn, experience and (hopefully) achieve while we're here. So, if you're on a tight deadline, your learning process may be accelerated. So, results may vary.
In my personal experience, however, the information comes primarily in two channels: reflectively, and via precognition. Reflectively--it's hard to summarize this, but frequently I have the distinct sense that "someone" is reminding me of random events from my past, almost as if to say, "Remember ten years ago when you did such-and-such?" It is always expressed as an open-ended question, with no judgement. But I always see a connection to my current predicaments.
Again, how do I know that these promptings aren't just my subconscious? I really don't know. But these past experiences are carefully selected, never random, and they are almost always things that I haven't thought about in years. There seems to conscious, deliberate intent behind the process. If I were simply neurotic (is "neurosis" included as a disorder in the DSM-5?), I would simply obsess over the same series of things over and over, and see no linkage with my present life. My experience is not like that.
Precognition is not as frequent, but it usually involves a certain "knowing" of what choice, action, or task to perform, in a certain way--to avoid harm, to myself and others, or to benefit others or myself. Most people recognize this as an "inner voice," or "something tells me I better do this or not do that." After having several of these experiences, I decided to take another look at the subject of spirit guides.
Update: I actually posted this entry Monday but pulled it down because I didn't like what I'd written. But I mentioned that I would try one of the techniques in the book. I tried the "Rapid Start Technique," specifically, expressing the question, "What is the one thing I can do tomorrow to improve my karma?" Yeah, I know--very New Agey. Once expressed, I actually went to sleep and forgot about it (which is always a given with me). I did not expect anything. But today (Wednesday) something happened out of the blue that caused me to dig up some memories and records from twenty years ago. It involved an extended family connection, a death, a funeral, as well as other things I had totally forgotten about. "This is odd," I thought. Then I remembered--my "rapid start" question. Was this an answer from a "spirit guide"? Maybe. Maybe probably?