.calt code model?

Sorry for my ignorance …
I just fail to find a suitable reference for the following issue:
The font features some peculiar descender shapes which collide with other letters in some instances. I’d like to solve this by the .calt feature. It’s only a few pairings, nothing really tricky I suppose. But I just miss to catch the suitable code model snippet … single glyph approach preferred over class definitions, probably.
– Any advise highly appreciated!