How were the first two stone tablets that God gave Moses inscribed?

Answer: By the finger of God

On Mount Sinai God gave Moses the two tablets of the Testimony, tablets of stone inscribed by the finger of God.

“When the LORD had finished speaking with Moses on Mount Sinai, He gave him the two tablets of the Testimony, tablets of stone inscribed by the finger of God.”— Exodus 31:18 (BSB)