Why quarantine new fish? Simple...to prevent outbreak of disease in your tank by observing the fish and if needed administer the proper treatment to the fish.
Usually quarantine with 0.3% salt in your water for about 3 weeks to a month. For first 1-2 weeks don't feed. Water changes are made once a day,about 50% is changed,but make sure when making water back to 0.3% salt don't forget that not all the salt is taken out with water you changed and add full amount of salt,otherwise you get ham yu! Some people forget this by mistake and fish die due to too high salt percentage. Also,after 2 weeks,you can slowly decrease salt percentage,which means don't need to add salt after water change for next week or so until quarantine period finish,then your fish should be safe to inroduce with your current stock.
This is only a general idea and one of the methods of quarantine. Perhaps some other bros may have diff view how to quarantine the fish.