Well the issue here is you need a way to test layouts so you will need game time to do so.
That being said you can get your feet wet just working with existing addons for the general stuff. You don't have to start by coding anything. Use addons that in game configs to get started. Learn the ropes of layout and design and the quirks that come with it.
You can of course dive right in and go right for addons that use Lua configuration. It is entirely up to you.
