Are the Anthony Hughes YouTube videos from a few years ago still relevant to the latest Dorico version?
Yes, in general the workflow of how you create and work with master pages is pretty much unchanged. Those videos remain an eminently decent guide.
The only differences you might encounter to my knowledge is where options are available in the UI - you used to activate a slider in the left panel to e.g. access frames to draw in, but now that’s a button in the toolbox on the left. There are also additional options available, such as importing/exporting master pages.
I started with a New Empty Project. I created one Player for Guitar, made a new Master Page Sets entry with some distinctive features and called it Guitar Part layout. I set the Master Pages Current Set to Guitar Part layout.
Then I created a new Player, the clarinet. I would like for the clarinet part to look identical to the Guitar Part layout. What is the easiest way to do that?
Right-click on any layout in the right panel of Setup mode and click Propagate Part Formatting. Then click the Guitar Part in the left side of the dialog, the Clarinet part in the right side of the dialog, ensure that both boxes at the bottom are ticked and hit OK.
If you’re intending to create further parts in this project, it might be quicker to go to Layout Options, select the Clarinet Part in the right side of that dialog, ensure that the Layout Type dropdown (bottom of the dialog) is set to Part, then click the Save As Default button. This will apply to part layouts in this project and in all new projects you create (until you save some new defaults or restore factory settings).