The following is taken from a Honeywell manual for the B777 FMS system. Fairly recently practised this in the sim - it is a bit startling initially (so many pages you are used to using are suddenly gone) but manageable.
I think BOAC, posting above, may have confused FMS with CDU - they are not the same thing. The CDU (boxes in the cockpit) controls inputs to, and feedback from, the FMSs (bigger boxes downstairs in the E & E bay).
Boeing 777 Flight Management System
ALTERNATE NAVIGATION
If both FMCs fail, .......
Initializing alternate navigation does not require any pilot action. It occurs automatically 2.0 seconds after the last FMC fails. Alternate navigation initializes using the last flight plan and last set of navigation radio frequencies downloaded from the FMC prior to FMC failure. Alternate lateral guidance is not enabled for an additional 3.0 seconds after alternate navigation initializes. Alternate navigation radio tuning is available as soon as alternate navigation initializes.
All CDU calculations are based on a great-circle course between waypoints. The CDU does not accept undefined waypoints or conditional waypoints. Complete departure or arrival/approach procedures cannot be manually entered or crossloaded from the FMC if they contain undefined or conditional waypoints. The CDU creates a discontinuity between such waypoints. However, individual legs of a procedure can be manually entered or crossloaded if they constitute a great-circle course.
Route changes are made on the ALTN NAV LEGS page the same as with normal FMS operations. All courses between waypoints are direct routes. Modifying the active waypoint computes a present position direct course. A route change to any one CDU crossloads the route change to the other CDU when the change is executed.
Only the active waypoint course can be referenced to magnetic north because the ADIRU can provide magnetic variation only for present position. All subsequent waypoint courses are displayed as true courses.
The radio must be manually tuned on each CDU in alternate navigation. The left CDU tunes the left VOR, DME, ADF, and left and center ILS. The right CDU tunes the right VOR, DME, ADF, and right ILS. In all cases, manual radio tuning is done on the ALTN NAV RADIO page.
The alternate navigation system operates from three CDU pages:
ALTN NAV LEGS page
ALTN NAV PROGRESS page
ALTN NAV RADIO page.
These pages are the only ones available in the alternate navigation mode. Executed flight plan modifications made on one CDU ALTN NAV LEGS page are displayed on the other CDUs