[lproven]

Well, here's a story about how to configure and use one that I wrote last year:

https://www.theregister.com/2022/01/07/foss_fest_foreign_let...

It's dead easy.

You type compose, then 2 (or very occasionally 3) characters which, put together, make the letter you want.

Compose, a, ` gives à. Compose, A, ` gives À.

And so on. Y,=, ¥. c,/,¢

You can usually guess the combination you need.

[tgflynn]

Yeah, that might work with a default keyboard layout but it doesn't work with a polytonic Greek layout. How would you type a lowercase alpha with an acute accent and a macron that way ?

[lproven]

I don't speak or write Greek, so I don't really have any insight at all into this. (I can just about, very poorly, read the alphabet, that's all.)

So I can only guess:

Let's assume we have a Greek layout. And that that has keys from alpha to omega, and a RightAlt or something, and keys that have some useful resemblance to acute, grave, circumflex, rough, smooth, etc.

Compose, type an alpha, type an apostrophe?

As an example, I sometimes type in Czech.

č is comp, c, < š is comp, s, <

Others...

î is comp, i, ^ ï is comp, i, "

ç is comp, c, comma

All that's needed is a vague visual resemblance to the desired diacritic.