First put up a display, blank screen is fine. Then define the keypad, with null keys. Gradually build the display and add functional keys. Keys are displayed from their icon index packed 4/word. Icon editor provides the index.
Save after you've typed more than you care to lose.
Take advantage of the continually refreshed display. It's there to provide motion. If you look at a dump of graphic registers, some are changing. Neat!
If you exceed a half-dozen blocks of code, think about factoring. If the application has several phases, they can overload one-another. Compile time is imperceptable.