Add dextrose to your beer or cider kit as per the instructions.  Usually you will need 1kg per kit.  You can also use dextrose to carbonate your beer in a bottle.  Simply add it to your beer prior to bottling.  Rule of thumb is 4-5oz per 5 gallon batch to get roughly 2.4 volume of CO2.  Also used for thinning out a beers body and increasing the alcohol content.