This word "himself" refers to David and is used to place emphasis on him and what he said. Alternate translation: "It was David who" (See: [[rc://en/ta/man/jit/figs-rpronouns]])
This means that he was inspired by the Holy Spirit. That is, the Holy Spirit directed David in what he said. Alternate translation: "inspired by the Holy Spirit" (See: [[rc://en/ta/man/jit/figs-idiom]])
Here David calls God "The Lord" and calls the Christ "my Lord." This can be written more clearly. Alternate translation: "said about the Christ, 'The Lord God said to my Lord" (See: [[rc://en/ta/man/jit/figs-explicit]])
Jesus is quoting a psalm. Here God is speaking to the Christ. To sit at the "right hand of God" is a symbolic action of receiving great honor and authority from God. Alternate translation: "Sit in the place of honor beside me" (See: [[rc://en/ta/man/jit/translate-symaction]])
In this quote, God speaks of defeating enemies as making them into a footstool. Alternate translation: "until I completely defeat your enemies" (See: [[rc://en/ta/man/jit/figs-metaphor]])