Hi Bshall,
If you enter the screen designer on a stock screen, you will be prompted to either enter read-only mode or create a copy for customization. Just like in classic, customization cannot be performed on stock screens. You must first create a copy and re-direct to it.
Our documentation on Method New customization is a lot more extensive than classic. I suggest you have a look at it in the Method New Help Center starting with Solution No. 441. I hope this helps.
--Audisho