Hi there stylis,
its simple multiplication and its dependent on what beer you are brewing. So as a rule of thumb its suggested that 5g sugar per 500ml bottle is a good starting point. So fro a 23L brew you will need somewhere around 230g of sugar. The recommended way to prime your beer is to bulk prime in an FV with a tap at the bottom. The idea is to put your priming sugar into the the bottling bucket then add the beer from another FV, this will mix it equally throughout the brew then you can either put into bottles via the bottom tap or put it int a pressure Barrel.
Lagers require more priming sugar and other beers maybe a little less. This is entirely dependent on your taste so you need to experiment a little.
if you are priming straight into the bottles then you have a harder job and if you are not using all the same sized bottles then it gets more complicated again.e.g. a 2L bottle will require 20g/bottle etc...
Hence the use of an FV for bottling.
{edit} - once primed and bottled you need to keep the beer warm for anywhere between a week and 2 weeks, this allows the yeast to convert the priming sugar into alcohol and CO2, the CO2 will sit above the beer at this point in time. Then you need to move the beer into the cold, this will force the CO2 into suspension in the beer and give you conditioning, again for a couple of weeks but its good practice to keep your beer cold or cool until you are ready to drink.