From the website:
the United Kingdom, it is the House of Commons that holds the power of initiating an impeachment. Any member may make accusations of any crime. The member must support the charges with evidence and move for impeachment. If the Commons carries the motion, the mover receives orders to go to the bar at the House of Lords and to impeach the accused "in the name of the House of Commons, and all the commons of the United Kingdom."
The mover must tell the Lords that the House of Commons will, in due time, exhibit particular articles against the accused, and make good the same. The Commons then usually selects a committee to draw up the charges and create an "Article of Impeachment" for each. (In the case of Warren Hastings, however, the drawing up of the articles preceded the formal impeachment.) Once the committee has delivered the articles to the Lords, replies go between the accused and the Commons via the Lords. If the Commons have impeached a peer, the Lords take custody of the accused, otherwise custody goes to Black Rod. The accused remains in custody unless the Lords allow bail. The Lords set a date for the trial while the Commons appoints managers, who act as prosecutors in the trial. The accused may defend by counsel.
The House of Lords hears the case. The procedure used to be that the Lord Chancellor presided (or the Lord High Steward if the defendant was a peer). However since the Lord Chancellor today is no longer a judge, it is not certain who would preside over an impeachment trial today. If Parliament is not in session, then the trial is conducted by a "Court of the Lord High Steward" instead of the House of Lords (even if the defendant is not a peer).
The hearing resembles an ordinary trial: both sides may call witnesses and present evidence. At the end of the hearing the lords vote on the verdict, which is decided by a simple majority, one charge at a time. Upon being called, a lord must rise and declare "guilty, upon my honour" or "not guilty, upon my honour". After voting on all of the articles has taken place, and if the Lords find the defendant guilty, the Commons may move for judgment; the Lords may not declare the punishment until the Commons have so moved. The Lords may then decide whatever punishment they find fit, within the law. A royal pardon cannot excuse the defendant from trial, but a pardon may reprieve a convicted defendant. However, a pardon cannot override a decision to remove the defendant from the public office they hold.
Similar to the British system, Article One of the United States Constitution gives the House of Representatives the sole power of impeachment and the Senate the sole power to try impeachments. Unlike the British system, impeachment is only the first of two stages, and conviction requires a two-thirds vote. Impeachment does not necessarily result in removal from office; it is only a legal statement of charges, parallel to an indictment in criminal law. An official who is impeached faces a second legislative vote (whether by the same body or another), which determines conviction, or failure to convict, on the charges embodied by the impeachment. Most constitutions require a supermajority to convict. Although the subject of the charge is criminal action, it does not constitute a criminal trial; the only question under consideration is the removal of the individual from office, and the possibility of a subsequent vote preventing the removed official from ever again holding political office in the jurisdiction where he was removed. Impeachment with respect to political office should not be confused with witness impeachment.