How does Emily Dickinson describe death as an experience she is looking back on in the poem "Because I could not stop for death?"

It seems that you are asking about how Dickinson characterizes death. In that regard, she creates him as her carriage driver, conveying her to her next state of being or non-being. Some readings have characterized death as a suitor. In either case, he is gentlemanly and patient -- a depiction that is emphasized by the perceptions that he "kindly stopped," "slowly drove," and "knew no haste." The narrator reciprocates his politeness by "[putting] away / [Her] labor and [her] leisure too..."

The carriage is the vehicle in which the narrator is able to revisit the life that she has lived:

We passed the School, where Children strove

At Recess - in the Ring -

We passed the Fields of Gazing Grain -

We passed the Setting Sun -

"The School" and the "Children" symbolize childhood. "The Ring" could indicate the circle of life, which nears its end with "the Setting Sun," or the onset of old age.

There is an indication that the narrator is being wedded to Death:

The Dews drew quivering and Chill -

For only Gossamer, my Gown -

My Tippet - only Tulle -

She wears a "gown" made of "tulle," not unlike the material of a wedding dress. They then "[pause] before a House that seemed / A Swelling of the Ground -". This indicates a grave, as a mound of recently dug dirt would create a mound. "The Cornice - in the Ground" parallels a tombstone. 

It is significant that they merely pause at this place. This action indicates that both will continue on. After all, the carriage also holds "Immortality." Thus, Dickinson regards the event of death as one stop on the long, eternal path:

Since then - 'tis Centuries - and yet

Feels shorter than the Day

I first surmised the Horses' Heads

Were toward Eternity - 

It has been centuries since the narrator has died, but it does not feel like very long ago. The "Horses' Heads" are those which powered the carriage toward "Eternity." "Eternity" could be death as a state of permanence or, given Dickinson's spiritual and religious inclinations, it could be the "pause" or "stop" on the way to spiritual "Immortality."