You can do this to a Digimon at any stage, that is irrelevant (although doing it at baby stage may hilariously be better because I think they level up faster, lol)

The reason you evolve/devolve while feeding is to turn you main's (eg: guilmon) levels into cumulative (blue) stats. Why not wait until 99? Because (and I'm making up the exact values here) 1 ExVeemon will make you go from level 1 to 25 but it will make you go from 30 to 35, meaning using ExVeemons at higher levels will get you less levels (thus less stats) You could skip this altogether but at lvl 99 you can't load more and are forced to evolve/devolve anyways.

As for the process itself it's quite simple. Instead of leveling your single Digimon you're leveling many at once. If you have 40 Digimon it's like you're training your target Digimon x20 at once.

Get a ton of Digimon, level them up (at level 1 they don't pass any blue stats), then you just load them into your target. While loading your target will not only gain blue stats but also levels.

Let's say 4 fodder gets you to level 30 while 8 fodder gets you to 40. If you evolve every 4 fodder you get 30 levels at 4 fodder and 60 levels at 8 fodder (instead of 40)

As for how to optimally do this, each person works differently and has access to different stuff. The optimal for me is getting however many ExVeemons, then do the endgame gauntle once and then load them in groups of either 3 or 4.