Inkay is a unique Pokémon, and that won't change in Pokémon Go. When it comes to the evolution of this Pokémon, you'll need to make sure you meet a specific requirement, which will be very different from almost all previous Pokémon. This guide explains what you need to do to evolve Inkay into Malamar in Pokémon Go.
You will need to make sure you meet the 50 Inkay Candy requirement before attempting to evolve the Inkay. Without those 50 candies, nothing will happen. You can earn those candies by catching more Inkays, giving them rare candies, or walking around to unlock more.
Once you have enough Inkay candy, the next step is to flip the phone over while Pokémon Go is activated. You may need to enable automatic screen flip in your phone's custom settings before you can do this. With your Pokémon Go app upside down, you can now evolve Inkay.
The only way to evolve Inkay is to do it while the phone is upside down. Otherwise, you will be forbidden to evolve it into Malamar. The animation for Inkay's evolution should be the same and you can probably get your phone back to normal when it evolves to check out your well-deserved Malamar.