Hang up the green sheet wherever you want your virtual TV set, using strong tape on the back of the sheet. Make sure there are no wrinkles in the sheet, because they will show up on your video when you record it -- making your virtual set look unrealistic.
Set up anything else you would like in your scene, such as an anchor desk or couch in front of the green sheet.
Make sure there is nothing green in your scene except the sheet hanging up in the background. For instance, if one of the people in your video is wearing a green tie, the tie's surface will be replaced by your virtual TV set when you edit later.
Record your television show. When editing later, replace all the green in the scene with your virtual TV set. This makes it appear as though your video was recorded somewhere else.