If you need to enter exact waypoint coordinates in a flight plan, which is pretty common when coding North Atlantic Tracks (NATs) and other oceanic routes, here's how to do it: 

On northern hemisphere: (00N-90N)
Our example is on 60N:
7 letter coordinates : 5 letter ICAO fix
60N180W-60N100W : 60N80-60N00
60N099W-60N001W : 6099N-6001N
60N000E-60N099E : 6000E-6099E
60N100E-60N179E : 60E00-60E79

That goes for all latitudes on the northern hemisphere. So we are on the
60N, starting at 180W (60N80) and moving to the east ending in 179E (60E79).

On southern hemisphere: (00S-90S)
60S180W-60S100W : 60W80-60W00
60S099W-60S001W : 6099W-6001W
60S000E-60S099E : 6000S-6099S
60S100E-60S179E : 60S00-60S79

Such encoded waypoints (i.e. 60S79) can be also typed into standard navigation computers, i.e. 737 FMC.

Thanks to Johann M. Kjartansson for preparing the instruction.