Not to put too fine a point on it, but, if we have an usurper in the office, we do not have a President. Therefore, just how do you impeach a person sitting in the office who is not a President when only the President can be impeached?
Just as if you hired a CEO who lied on his resume, and your Stockholders had a procedure for removing the CEO, the same procedure would be used to remove an unqualified CEO as a qualified but incompetent or criminal one.
“In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the same shall devolve on the Vice President” U.S. Constitution from Article II
And say Hello to President “brain damage” Biden.