What is the best way to learn Substance 3D Designer?

Everyone learns differently so the main thing is to embark on a journey in a way that you will enjoy and work for you.

I would probably start in 3D Painter, and use downloadable models from Adobe 3D assets and standard materials. That will give you a feel for applying texture and material.

Next you could move to 3D Stager to set that model up against a background and render it.

Finally you could look at 3D Designer / Sampler for creating your own materials. The difference being that Sampler is good at creating materials from real images, Designer for creating entirely procedurally.

There are many tutorials around and the Adobe team have created some good ones here to get you going. Start simple, ask if you get stuck, and above all - enjoy the journey.

https://substance3d.adobe.com/tutorials/courses/Start-3D/youtube-J9t7PVw8vLQ

https://substance3d.adobe.com/tutorials

Dave