Depending on what you do - writing it down can be a first step. This way you can streamline it after you put in everything.

Generally it's a hard thing because different topics and audiences have different requirements. With experts you can talk differently and talk less straightforward because they know what to expect and how things work.

Personally I spent like a decade in a debating society and had to learn to somewhat order my thoughts. Having a rough outline on what you want to say is already extremly helpful. Sorta making out important things in a specific order you want to cover. Things you want to skip over. Thinking if you want to have some arc in the story if you want to tell events exactly as they happened...

There are a bunch of ways, but in the end the core idea is the same: know everything you want to say BEFORE you start. Either in your head, or with some notes - can be just words, can be short sentences, can be a graph/mindmap. Either way, ordering thoughts needs you to "grab" them first. Maybe later on it will become more automatic, but that takes practice.