TL;DR of video – Set up GitHub with your UnrealEngine.com account. Get your invite and fork the repo then clone it to your machine. Add the EpicGames repo as a remote to pull the latest changes (you can then push the latest changes to your fork). Run the Setup.command then the GenerateProjectFiles.command scripts to get all the files that didn’t come with the repo (either downloaded or generated). Open up XCode and compile.
Along with access to the Unreal Editor, a subscription to Epic’s Unreal Engine also includes access to the source code which would allow you to download and build the Unreal Editor/Engine from scratch.
Warning however, this is a completely unnecessary task if your desire is purely to poke around and see if Unreal is the engine for you. If you can’t say with certainty why you’re trying to compile this code then you probably shouldn’t be; at least not yet.
There are many reasons to desire doing so and I leave it to you to decide what that reason is.