First, the easiest way: have the narrator tell it. Doing it that way is easy and effective.
From The Bat by Mary Roberts Rinehard and Avery Hopwood
The problem with adding back-story this ways is clear. It's not exciting or original - it lacks drama.
Let's look at another way to make it more dramatic and original.
Disclosure through dialog:
Here's an example from Careless in Red by Elizabeth George:
The key is - never let the back-story sound forced in dialog. Don't bring up things in the past if they wouldn't be uttered otherwise. When we're asked questioned about our past, we won't start at birth and explain our lives and what happened until this point... in fact, most won't disclose more than a line or two unless prodded. Do the same with your story.
Tomorrow, I will disclose other techniques.