For adults, I usually use number system.
For example, everything starts in C major first. In solfege, C will be Do, D as Re, so on and so forth. So Do will be number 1, re as 2, so on and so forth.
In my music schools, we have some collection of songs, basically pop tunes, church hymns, etc, which we have simplify most of them into C major (or A minor). And they are in big notation.
So, I simply teach a song by just writing the number above the notes, with some indication on the solfege. Viola, they can do tunes like "Memory", "pass it on" in their first lesson.
Scales, and much more demanding classical repertoires only comes in if the adult student is determined, obeying (at least listen to what I say), and show appreciation in classical music.
I also create some simple five fingers exercise, which most of my students do b4 the lesson. Some may not want to do it, as they are somewhat boring. But actually I fing that these exercise helps my student to familiarize with the geography of the keyboard better. It helps them to pick up a song much faster as well.
Anyway, teaching adults has never been easy. You may say that I have given up on most of my adults students. I just teach for the sake of money.
