Persistence, mostly.

Learning the word 'in' and what pointing at the other room/snapping fingers means was the first step. Took a while, but enough picking up/putting down, blocking and gentle scooting got the point home.

After that, get them 'in' and start making food on the counter, not direct to bowls on the floor. When a cat crosses the boundary, stop serving up and put them back in either by command or physically. Be belligerent about it if you need to be. Resume/repeat until everyone's dishes are ready.

When ready, I call the first one in by name and put bowls down. The rest quickly follow.

You may find that the cat with the most bossiness/brains/food drive helps keep the others back on pain of biffing, cause they know intrusion means food delay.