At the end of the 1970s when I was a Leading Railman (Numbertaker) at Andover, I used to do overtime by working Sundays on the platform (*). On the early shift the only other member of staff was the ticket collector in his box so I had to do the announcements. It was quite simple: pick up the microphone, hold it about 8-10” from your mouth, press the button with your thumb and speak. We just announced the main calling points and a couple places you could change for at Salisbury or Basingstoke: nothing about buffet facilities, First Class, ticket validity or anything else. If necessary we would give out information about delays.
We weren’t trained to be announcers or selected because of our beautiful elocution, it was just a minor part of various posts. The people who did it had a variety of accents, including one or two thick Hampshire burrs. Training just consisted of being shown the tannoy units, told how far to hold the microphone from your mouth, and doing a Testing-One-Two-Three-Four so whoever was teaching you could listen and check you weren’t getting feedback: one minute, at most.
* - The two two Railman were old boys who were rostered to do every Sunday, but were happy to give up around half of them. The shunter and myself, both on flat weeks, got them instead.