The impeachment procedure is in two steps. The House of Representatives must first pass "articles of impeachment" by a simple majority. (All fifty state legislatures as well as the District of Columbia city council may also pass articles of impeachment against their own executives.) The articles of impeachment constitute the formal allegations. Upon their passage, the defendant has been "impeached."

Next, the Senate tries the accused. In the case of the impeachment of a President, the Chief Justice of the United States presides over the proceedings.
Why isn't this happening?