preface (something) by (doing something)
To do something as an introduction to something else. Often used in passive constructions. Let me preface this statement by first apologizing unreservedly for what has happened. She prefaced her speech by reading well wishes written friends and family from around who couldn't make it to the wedding. The ceremony was prefaced by a performance by the school choir.
preface something by something
to begin something by saying, writing, or reading something. I would like to preface my prepared remarks by making a personal observation. Her remarks were prefaced by the reading of a poem.