I'm glad you recognize that there isn't a path yet; many people don't!
The short answer today is: you have to ramp up on creating content in 3D engines. If you can't build out all of the content yourself, you can't effectively work. That means, to some degree, creating original 2D and 3D assets. The #1 thing Microsoft tells designers who want to work with Hololens, is learn Unity and learn the workflow to deploy Unity content into a Hololens yourself, without developer/engineer assistance. (UnrealEngine4 is the other major option.)
As a web-based designer, tools like React VR and A-Frame might be useful, but your tooling there is always going to be a distant second to the established 3D players, and so your content quality will also lag behind.
YC's guide: https://blog.ycombinator.com/how-to-get-into-vr/
Some other Medium post that wasn't bad: https://blog.prototypr.io/designing-for-vr-a-beginners-guide...
As far as using real hardware, your AR options are kind of limited if you can't afford a Hololens. You could try and find a second-hand CastAR Kickstarter developer prototype headset. You could maybe fake something with the Vive's front-facing camera or the Leap Motion as a camera on an Oculus Rift. You could cobble together a Pepper's Ghost-style Google Cardboard-alike, like Aryzon: https://www.kickstarter.com/projects/aryzon/aryzon-3d-augmen... or this DIY version: http://www.instructables.com/id/Cardboard-Hololens/
But, mostly, of course, you have to have something to build, and blog posts won't help you there! There isn't a curriculum of standard AR/VR exercises like there basically is for web-based design (build a landing page, build a CMS admin interface, build a shopping cart, etc.).