1) Fill tank (*), reset trip recorder or note mileage.
2) Do some driving (the further you go until refuelling the better).
3) Fill tank (*), note the distance (miles or km) you did since the last fill and the amount of fuel (litres) you've put in.
Calculations:
Conversion factors: 1 mile = 1.609 km, 1 imperial gallon = 4.54 litres
a) consumption in kilometres per litre = distance (km) / fuel (litres)
b) consumption in litres per 100 km = 100 / (consumption in kilometres/litre)
c) consumption miles per imperial gallon = distance (miles) / fuel (imperial gallons)
(*) Be consistent when refuelling: stop filling the first time the pump cuts off, for example.
For example, my last refuel was at 360.5 miles (580.0 km) since the previous fill.
I put 40.45 litres fuel in.
a) consumption km / l = 580.0 / 40.45 = 14.34
b) consumption l / 100 km = 282 / 14.34 = 6.97
c) consumption MPG = 360.5 / (40.45 / 4.54) = 40.46
(Aside: not bad for a Cooper!)