Here is a in-depth guide to on how to evolve Inkay in pokemon go.
First, make sure you have an Inkay in your Pokemon collection. You can catch Inkay in the wild by using a Pokeball, or you can hatch an Inkay from an egg by walking a certain distance.

To evolve Inkay, you will need a certain amount of Inkay candy. You can earn Inkay candy by catching more Inkay in the wild, transferring Inkay to Professor Willow, or hatching Inkay from an egg. You will need 50 Inkay candy to evolve Inkay into Malamar.
Once you have an Inkay and enough Inkay candy, tap on the Inkay in your Pokemon collection to open its profile page.
On the profile page, you will see a button labeled “Evolve.” Tap on this button to initiate the evolution process.
NOTE: Before you tap the button, turn your phone UPSIDE DOWN, otherwise the Inkay WILL NOT evolve!!!!!

A confirmation screen will appear, asking if you are sure you want to evolve your Inkay. Tap on the “Evolve” button to confirm your decision.
The evolution process will take a few seconds to complete. During this time, you will see a progress bar and a message indicating that the evolution is in progress.
Once the evolution is complete, you will see your newly evolved Malamar. You can check out its new stats and moves by tapping on the Malamar in your Pokemon collection.

Keep in mind that evolving a Pokemon costs a certain amount of Stardust and Candy, so make sure you have enough before you try to evolve your Inkay. Also, keep in mind that once a Pokemon is evolved, it cannot be reversed, so make sure you are ready to commit to the evolution before you begin the process.