How "deep"  into panel design do you want to get?

Technically, armed with only a basic text editor and the Panel and Gauges SDK you should be able to modify panels by moving gauges around, changing bitmaps and such.

As usual, there are programs and utilities that will help.

The serious panel builder will find it worthwhile to purchase FS Panel Studio to add the graphical interface to "drag and drop"  gauges, etc.

If you have FS-X Deluxe, and have the SDK installed, there's even a basic gauge maker utility.

....mmmm......can't see that you need anything sophisticated to do that......As Felix asks "How deep do you want to go?"

On a basic level just study the contents of the panel folder and swap things around to suit.

....make new bitmaps from photos of the actual panels you have seen in your real a/c making them the same size etc as the default ones, then in the panel.cfg move the default gauges around and resize them to fit where they need to be on your new bitmaps........also "borrow" more gauges from other a/c and delete any you don't need.........

Like I said, study the files in the panel folder...  it is hardly rocket science to see how it works.........try things out, experiment........you'll soon get the hang of it.............commoner