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).