In a nutshell, once you have macOS Mojave installed:

• Bring up System Preferences
• Select Desktop & Screen Saver
• Click the Desktop tab
• Find a picture with the dynamic desktop icon (a circle with a parabola in the middle) in the upper left corner. Click to select.

As of beta 2, there is only one dynamic desktop set, the one they demoed in the WWDC keynote. But other people have successfully created their own, and seems likely to me we’ll see more before the final Mojave release.

As always, this is beta software, so backup before you install, then proceed with caution.