Step 1: Make a background image on your laptop
I made this background in Photoshop, but you can use anything you like.
Save it so you can fill the screen with it in the next step.
Step 2: Put your laptop behind your subject.
Step 3: Photograph what you see with your camera
-- If you light the subject, stop it spilling onto the screen by choosing the direction carefully (e.g. no frontal flash).
-- Watch out for digital artefacts (patterns or lines) on the background if it is too sharply focussed (see also comments from others).