"spoke this message from Yahweh" or "declared these words of Yahweh." This is a formal way of saying that the words that follow come directly from Yahweh.
# by myself I have sworn
"I have promised and I am my witness." To swear means to use the name of something or someone as the basis or power on which the oath is made. There is nothing more powerful for Yahweh to swear by than himself.