Step 1: give the Pokémon that already has the move you want to pass on the Mirror Herb item.
Optional: delete other moves on the Pokémon that already has the move to avoid possible mistakes.
Step 2: free up a move slot on the Pokémon that is learning the move. It does not need to hold any specific item.
Step 3: host a picnic with both Pokémon in your party.
The move should now be known by both Pokémon.