To get started with developing you first need to install the latest version of the Unified Remote Server, and the full version of the app (or at least 1 unlocked remote). Note that this site only applies to the V3 server. Older versions of the server use a completely different API for developing remotes.
If you want to modify and existing remote, or learn how to create a custom remote, the best way is to look at all of the existing remotes, which are available on GitHub. We welcome all pull-requests if you would like to share your remote, or improve and existing remote.