Read a ruler, a set of scales and a measuring jug, then swap between units: 100 cm make a metre and 1000 g make a kilogram.
Count the big centimetre marks, then the little millimetre marks.
10 mm = 1 cm
Length
100 cm = 1 m
Length
1000 m = 1 km
Length
1000 g = 1 kg
Mass
1000 mL = 1 L
Capacity
kilo means a thousand: 1 km = 1000 m
centi means a hundredth: 1 cm is a hundredth of a metre
milli means a thousandth: 1 mL is a thousandth of a litre
Line the start of the object up with 0, or note where it starts. The numbered marks are centimetres and the 10 little marks between them are millimetres. Count the whole centimetres first, then the millimetres left over: 7 cm and 4 mm is 7.4 cm.
Find the two numbered marks the needle or the water is between. Work out what each small step is worth (100 split into 5 steps is 20 a step), then count the steps on from the lower number.
Learn the facts: 100 cm = 1 m, 1000 g = 1 kg, 1000 mL = 1 L. Going to a smaller unit you need more of them, so multiply; going to a bigger unit you need fewer, so divide. Multiplying by 100 moves every digit two places to the left.
Children measure in cm, g and mL at about 6 to 8. Reading scales with steps of 2, 5, 10 or 20, and changing between units, comes at about 8 to 11.