You need to first figure out the core of how your app will be different from others (but also what other more usual features you'll need to include to make it u

You need to first figure out the core of how your app will be different from others (but also what other more usual features you'll need to include to make it usable). For example, I'm working on a micro voxel game, but existing tools lean more towards artist needs, such as creating and colouring static voxel data sets (which can then be rendered etc). So while working on the game I'm also making an editor that will have more advanced features such as different types of animation, dynamic voxel scaling, support for larger scenes, lighting&shaders etc. So after figuring out what your app will do you'll need to figure out where/how it'll be used. Again, I'm coding in Unity, so I'm making the editor also run as a Unity app. You probably need to decide between online/offline, then figuring out a framework that will allow you to create your tool in. While figuring that out you also need to investigate how you'll eventually publish it etc.