The best tip is to follow the instructions as stated on the stain can. You can watch some YouTube videos or read a few articles to get familiar with the process but the manufacturers instruction are what you need to follow. You should try a test piece too and that may even be in the instructions.

As for the woodworking part, cedar fence pickets are extremely rough and likely will have a bow in them. For the look in the picture, you'll need a rough sanding to remove the burs/splinters and some jointing to remove the bow. I wouldn't be surprised if there was a 1/2 piece of plywood on the back with a 2" offset so you don't see the backing and give it some more of that floating look. Makes sense too as you can screw in through the back to secure the stained wood up front.