To evolve Onix into Steelix, you will need a Metal Coat as shown in the picture below. This guide shows where to find a Metal Coat and how to Evolve Onix into Steelix in Pokemon Brilliant Diamond & Shining Pearl (BDSP).
Where to Find Metal Coal
There are a few ways you can get a Metal Coat in the game. Below I will show two ways in which you can get a Metal Coat.
Get Metal Coat in Iron Island
You will get Metal Coat on the Iron island. But to get it, first, you will need to get the National Pokedex. To get National Pokedex, finish the Pokemon League, and register all 150 Pokemon to the Sinnoh Pokedex. Then go and talk to the Professor in Sandgem town to get the National Pokedex.
Once you have the National Pokedex, go to Canalave City. In Canalava City, take the boat to the Iron Island as shown in the picture below.
On reaching the Iron island, get down the boat and go upstairs. then enter the small house as shown in the picture below. talk to the man inside the house and he will give you a Metal coat.
Steal Metal Coat from Magnemite
You can also get a Metal Coat by stealing it from Magnemite. To steal, you will need ‘Thief’ TM. You will find Magnemite in the Grand Underground. Put Steel-type mods in your secret base to increase the chances of Magnemite spawning.
Catch an Onix
You will find Onix inside the cave on the Iron island. It has a 40% spawn chance there, so you will easily find one.
Evolve Onix into Steelix
Once you have an Onix and a Metal Coat, give the Metal Coat to the Onix. Then trade the Onix with another player. The Onix will evolve into a Steelix when it is traded while holding a Metal Coat. So basically, the other player with whom you traded will receive a Steelix. Click here to see how to trade Pokemon.