You can purchase a hydrometer here.
A hydrometer measures the amount of sugar in your wine / must and uses a scale called Specific Gravity (SG).
Firstly I would say that the readings are only ever fairly approximate (although reasonably accurate) as readings vary depending on temperature and you can’t get an exact reading anyway (a bit like the using a scale with a dial rather than a digital scale).
Also, when you are taking your initial reading of your must to calculate how much sugar to put in, the must is actually more concentrated as more water will be added with the sugar and possibly more again to top it up.
a/ Sterlise the hydrometer and the sleeve it comes in (or your test jar), in potassium metabisulphite
b/ Pour some (room temperature) must into the sleeve (I keep a little milk jug sterilised for this purpose). You may need to strain a small amount of must to do this. Don't forget to sterlise your sieve.
c/ Spin the hydrometer in the must until it stops (there should be enough must for it to float)
d/ Take your reading.
e/ Note that the lower the SG, the further towards the top of the hydrometer the figure is. This is counter-intuitive for some people. See the pictures to the right the first reading of 1.046 is further towards the top of the hydrometer than the second reading of 1.074
For wine you usually want your SG before fermentation to be between 1.075 and 1.100 (170-220g sugar per litre) (more and you will find difficulty getting your ferment started), this should give you a wine of about 10-14%. A wine of less than 10% will have reduced keeping ability and may need more preservative.
e/ Use the figures on the hydrometer to calculate how much sugar to add eg if the SG of your must before adding sugar is 1.050 (100g sugar per litre) then add 120g sugar per litre (about 600g per 5 litres). Email us on email@example.com if you need some help working this out, it can be confusing until you get the hang of it.
f/ Check the SG once you’ve added the sugar and see if you have the required SG
g/ Check the SG when you think your ferment is finished. A finished ferment should have an SG of 1.000 or lower.