To build your project using IL2CPP, open the Build Settings window (File > Build Settings). Select the platform you are building for, then click Player Settings… to open the PlayerSettings window in the Inspector.
In the PlayerSettings window for your target platform, scroll down to the Configuration section. For Scripting Backend, select IL2CPP.
With IL2CPP selected as the Scripting back end, click Build in the Build Settings window. Unity begins the process of converting your C# code and assemblies into C++ before producing a binary file for your target platform.