What is the best way to transition into a flashback scene when writing fiction?

Figure out a way. Like when you write your hero into a corner then have to get him/her out. Do something no one else has thought of.
In the TV show “Kung Fu” there are fadeaways, not really flashbacks, to show Caine’s past experiences (sub-plot) with the Shao Lin and how those experiences influence how he proceeds in the (present) plot.
Write the flashbacks like a intro to each new chapter, like the intro to an entire book.

It really depends on how extensive the flash backs are. From your own summary, it seems that an italicized introductory sentence to a whole separate chapter would work well.

@NerdyKeith felt the cool mist on his skin, and wondered if it would moisturize his teen age face, dried from the peat fires in his Donegal cottage…

@kritiper Great advice, perhaps a fresher approach to flashbacks would be best for my book. Thank you.

A possibility might be to alter the point of view.

If the base of the story is third person narrative, your flashback scenes could be told in the first-person, either narrative or omniscient, depending on whether you want to take advantage of benefit of hindsight.

This would work best if all the flashback scenes involved the same character.

@Seek Yes that is great advice, however It will be multiple characters at different stages.

There is no “best” way; it’s a matter of style and convention.

I tend to prefer to make them explicit, to include awareness that a specific character is having this memory, and that the memory may not be perfectly accurate – it’s still a present experience.

I also try to avoid over-use of flashback and plots that loop back in time. As a reader, too much flashback and convoluted plot timelines seem often over-used. I’d rather fall forward through time, and have the flashbacks myself. (And no one does this, but I’d personally appreciate footnotes showing me where to find references to what’s being recalled in later chapters (or books in a series).)

@Zaku Thats a very good point, thank you so much for your advice. I agree.