Download the emulator from This App.
You will need a BIOS file. Now you won’t get this bundled with the app. Most developers are aware of the legal hassles involved with such files and so won’t include it with their app. Without it, you won’t be able to play any games on your emulator.
Besides installing the BIOS file, you will also need a ROM file of of your favorite game. (Make sure you have a genuine copy of the game.)
The file should have a “.nds” extension. The emulator supports .nds ROM format, but also has the ability to extract compressed archives such as .rar and .7z files.
You can move ROMs to a separate folder created on your SD card icon. You can use the phone's USB mass storage option to transfer game files.
Name your folder “NDS Games” to ensure ease in navigation. Next, open your emulator and navigate to the folder, find the ROM, and double-tap on it to play. You can control in-game characters via the virtual buttons.