A Committee of the whole House meets in the Chamber. All MPs are members of the Committee and any MP can take part. It considers the Bill in the same way as a Public Bill Committee, but doesn’t take evidence.
Before the Committee starts the Speaker leaves the Chair and a Deputy Speaker or a member of the Panel of Chairs (MPs chosen by the Speaker for chairing duties) takes over. The ceremonial Mace is removed from its place on the table in front of the Speaker’s chair.
Once this has happened the House is in Committee and the debate on the amendments and clauses begins.
There’s no need to write to the Speaker in advance to take part in a Committee of the whole House. If you want to speak, stand up when the previous MP has finished and try to catch the Chair's eye.
As in a public bill committee, once MPs have gone through the detail of the bill it is reported, with any amendments made to it, to the House for the next stage of its progress.