Beating the Elite Four and becoming the next Pokemon champion has never been the only goal of Pokemon games - you'll also want to catch all the Pokemon that fill your Pokedex, and Shining Diamond and Shining Pearl are no different. You can normally find Pokemon all over the world, either by using certain items or by trading version-exclusive Pokemon. However, some Pokemon require a special method to evolve, and Scyther is a great example.
Related: Where to find Reaper armor in Pokémon Brilliant Diamond and Shining Pearl?
To evolve your Scyther into Scizor and help you complete your Pokedex, you will first need the held Metalcoat item. Finding this article can be a little tricky. You will need to go to Canalave City and access the ship, then set sail for Iron Island. Once docked, enter the neighboring house and talk to Byron to get the Metal Coat.
With a Metal Coat now in your possession, you'll need to give it to Scyther to keep it. Now you will need the help of a friend you trust. Start a trade with your friend and send Scyther while he is holding a Metalcoat. Being traded while holding an Metal Coat will initiate Scyther's evolution into Scizor. Ask your friend to return the Scizor to you and you'll be one step closer to getting them all.