Yesterday, I noted that I had written an introduction for The Mysteries of the Faceless King, the first book of the two volume collection The Best Short Fiction of Darrell Schweitzer. I meant to also write a few words on the art of introductions for the sake of as-yet-unpublished writers who will be facing the chore themselves someday. Alas, I was getting some productive work done on a short story so I didn't have the time.
Today, if I keep my remarks brief, I may be able to patch something together.
Blurbs and introductions, if they are effective, must have two things in common: The must be complimentary and they must be true. It does less than no good at all to slap "Like P. G. Wodehouse on giggle gas!" onto the cover of King Lear. Which means you'll have to sit back and analyze not just why you like the work in question, but what the kind of people who will be happy they bought it are looking for.
This is even more difficult when you're writing an introduction--particularly for a "best of" collection--because the reader has already bought the book, probably read it, and almost certainly formed their own opinion of its virtues.
The solution to that problem I'll leave up to you. Each book, if it's worth introducing at all, is unique. So I'm afraid I can't help you there.
But I can help with the format.
Long ago, the late, lamented, and extremely useful editor Jim Turner asked me to write an introduction to a collection of stories by a writer I admired immensely. This may have been the first such I ever wrote. At any rate, I gave it my best and sent it to him. And he immediately sent it back to me.
"Don't be clever!" he told me angrily. "This isn't about you. It's about the fiction. Say something substantive about each story in the collection. Then stop."
So I wrote a new intro from scratch. I found something to say about the virtues of each story in the order they appeared. When I was done, I found that I'd described everything I admired about the writer's work.
It's as simple as that. You also have to come up with a beginning and an end to tie the whole thing together. But I'll trust you to take care of that little detail on your own.