Here "hand" represents the entire group of witnesses. Alternate translation: "The witnesses themselves must be the first ones to throw stones. Then all the people will execute the person by throwing stones at him" (See: [[rc://en/ta/man/translate/figs-synecdoche]])
The nominal adjective "the evil" can be stated as an adjective. Alternate translation: "you must remove from among the Israelites this person who does this evil thing" or "you must execute this evil person" (See: [[rc://en/ta/man/translate/figs-nominaladj]])