When inside the cockpit ready to fly, you can use the FMC to setup a simple route.
Use INIT button to get INIT page. Then click its other buttons to enter FROM/TO Airport name like EGLC/EGLL, it will show on the bottom left of the screen, then press top right button next to FMC screen to activate and see it on the screen there.
Then use F-PLN button and select EGLC and later EGLL to set DEPARTURE and ARRIVAL runways and ILS with optional waypoints. Then *INSERT on bottom right of screen. May need arrow button to scroll for more more options.
May get F-PLN DISCONTINUITY, when reach it in flight, must use DIR to jump the gap. Scroll to the waypoint beyond the gap you want, press left side button next to it. Then top left button next to *waypoint to activate.