In almost all the books I’ve read about affirmations, they are usually talking about communication with the subconscious mind, not the higher self (unless, of course, you are affirming to communicate with the HS).
Affirmations are just statements of what you want to be true, said in the present tense. “I am rich.” “I am powerful.” That kind of thing. They are usually said first thing in the morning, before going to bed, and when looking into your own eyes in a mirror. In order to better access the subconscious, it is usually recommended it be done in an Alpha state, which is why upon awakening and as you drift off to sleep are the usual times to say them.
The biggest issue most people have with saying affirmations is that of the belief factor. If you can’t pay your rent, you can say “I am rich” a million times and it won’t do anything because you just cannot make yourself believe in the statement enough to bridge the gap between where you are and where you want to be. Being in an Alpha state can help close that gap and drop the affirmation directly into the subconscious mind.
Affirmations work for millions of people. They also don’t work for millions of people.
As Eva posted, Afformations are something similar where you ask yourself questions why something is true, rather than trying to force yourself to believe in a statement, and then you take action based on the questions. I have read both of Noah St. John’s books on the method and while there is a lot of inaccuracy in what he says, and a whole lot of stroking his own ego, people have had success with using afformations instead of affirmations so it might be something to look into.
The “speak it into existence” thing you mention is part of the Law of Attraction hulabaloo, but it is not used the same way as an affirmation, by repeating a statement, but more like a verbal sigil, where you speak your intention infused with your will, and then completely put it out of your mind and forget about it.
I have had zero success with affirmations over the years but I did have one notable success with speaking something into existence.